There is some overlap, such as the Marvel and DC stuff and then their universes in other mediums, but that's just kind of how it goes.
1. The Marvel Universe - The first time I really got into something that wasn't a kid show to sell toys. My first Marvel Comic was Spectacular Spider-Man #199 (1993) and it featured a team-up between Spider-Man and the X-Men. I thought it was awesome and loved exploring every tiny corner of it. Huge fan of Spider-Man, the Fantastic Four, the Avengers, X-Men, Daredevil, etc.
2. The DC Universe - I discovered DC after Marvel. While I read the Green Lantern portions of some Action Comics I got once, my first real journey was in 1996 when I read JLA: Year One, JLA, and The Flash. I was a huge Flash fan from the start, got into Green Lantern, and of course Batman and Superman and Justice League.
3. The Buffyverse - Still my favorite television franchise of all time. Such a great concept, amazing characters, and crazy twists and turns along the way.
4. The Simpsons/Futurama - I decided to group them together since they're comedies. And what comedies! I quote the show so often sometimes I do it without even realizing what I'm referencing.
5. The DCAU - I maintain that the DCAU is the greatest adaptation of a source material ever. And the fact that it churned out new material for about 15 years actually makes it just as valid as any period in the DC comics Universe (which tends to restart every 10-20 years our time). Batman the Animated Series, Superman the Animated Series, Batman Beyond, Justice League and everything else...it was a thing of beauty that will take a LOT to top, if it ever happens.
6. Disney Animated Canon - Here I'm just talking about the 50+ movies, although I will include House of Mouse and Kingdom Hearts. My three favorite Disney movies are The Little Mermaid, Aladdin, and Hercules, and while I don't love every single movie (still working on the ones I haven't seen yet), they are tied together enough that I get excited when I think about them.
7. Vertigo Comics - Not exactly the DC Universe, but with Vertigo's storytelling, I feel like these are the best comic books have to offer. While I haven't read every Vertigo title, Sandman, House of Mystery, The Unwritten, and everything related to Fables remain favorites.
8. Hercules/Xena - Not as great as the Buffyverse, of course, but I loved both Hercules and Xena at the time. I was in high school and it sort of felt like my real transition into hour long dramas. Yes, it was cheesy, but it felt adult, and oddly enough, I still hold up shows I watch today to the level of excitement I felt back then.
9. The Marvel Cinematic Universe - It still feels young to me, but wow, I'm really blown away by what the first Iron Man movie did and how it spawned an amazing universe that is deep, fun, captures the spirit of the comics, and really does live up to the hype.
10. Gargoyles - I maintain that Gargoyles is the greatest animated series based on an original idea. It didn't talk down to the audience and Greg Weisman put a LOT of thought into characters and timelines and was able to incorporate so much mythology into it. A true thing of beauty.
11. ReBoot - Second favorite original animated series. I loved ReBoot as a kid, and thanks to its long production time, those 4 seasons lasted pretty much the entirety of the 90's and my enthusiasm lasted the whole time. Such a cool show, and things CHANGED over the course of the series (I'm still shocked at what they did with Enzo Matrix). Too bad it ended on a cliffhanger.
12. Teenage Mutant Ninja Turtles - Yeah, I was obsessed with it when I was a kid. Cartoon, toys, video games...hard to top that.
13. Disney Princess - Going off on my Disney movie love, I absolutely love the Disney Princesses. The Little Mermaid is my favorite, but Ariel really is a gateway princess, and I ended up really getting into the franchise (Jasmine is my solid second favorite, but my third tends to change between Belle, Mulan, Tiana, and some of the others depending). Luckily, as an adult male, I am immune to most of the merchandising, but wow. I don't care what anyone says, the magic and sheer optimism this franchise is bursting with makes me genuinely feel good about everything.
14. Final Fantasy - My favorite video game series of all time. I've only played VII-X (and Kingdom Hearts, which had some crossover characters), and I logged in a lot of hours in that series. My favorite video game series because I loved the stories, gameplay, and mythology.
15. Harry Potter - Favorite book series. I love the magic, the fantasy, and the fact that the characters grow and develop consistently.
16. The Avatarverse - Sadly since I watched this as an adult, it's going to have to be a solid third to Gargoyles and ReBoot. However, Avatar the Last Airbender tops those shows in one respect because it has a solid beginning, middle, and end. It's got a cool concept, characters that grow, and a definitive ending. Plus Legend of Korra is a great spinoff and actually tells you what happened to the major characters of the previous series (being set 65 years or so after A:TLA).
17. Star Trek - Star Trek is THE TV franchise. It may not be my favorite, but I appreciate the pop cultural icon status, the concepts it introduced, and the people it inspired. Star Trek: The Next Generation and Deep Space Nine are still two of the greatest hour longs I've ever seen. And despite the fact that it's a TV franchise, it's also one of the better movie series of all times (going by a "best of" standard).
18. Pixar - Probably the best hit-to-miss ratio of any movie studio I've regularly followed. Pixar movies are MUST SEE for me. They manage to include something in their movies for every single demographic, and that I can respect. Plus they know how to tug at the heart-strings.
19. The Disney Afternoon - Another kid fandom that I have to put on my list just because of how excited I would get to come home from school and watch the Disney Afternoon. Favorites include DuckTales, Darkwing Duck and TaleSpin.
20. The 90s Marvel Universe - Although you can find contradictions if you look hard enough, basically, X-Men, Spider-Man, Fantastic Four, Iron Man, and Hulk are all in a shared universe. And it's low budget, cheesy, doesn't hold up to modern day standards, but I love it all the same. A case can be made that Spider-Man Unlimited is included, but Silver Surfer and Avengers United They Stand are certainly not.
21. Nicktoons - Another thing I've been a fan of since childhood. Since Doug, Rugrats, and Ren and Stimpy. I separated Avatar out from that despite the fact that it's technically a NickToon. For me, NickToons are those classic comedies that I still think about and quote.
22. Transformers - Yeah, I was really into Transformers in the 80's. REALLY into them.
23. Dreamworks Animation - While I may love Disney and Pixar, I have to acknowledge the awesomeness that is Dreamworks. Most notably Shrek and How to Train Your Dragon. Dreamworks isn't exactly a must see for me, but I still love their movies.
24. Lost - I don't care what anyone says, Lost EARNED its finale. While I don't enjoy every individual episode as much as, say, Buffy, and the franchise isn't as spectacular as Star Trek, it managed to be one of the most satisfying series of all time.
25. Ultimate Marvel - I'm separating this from the mainstream 616 Marvel Universe. Ultimatum was horrible and killed a lot of excitement I had for it, but everything previously was golden. The Ultimate Marvel Comics were the best updating/reimagining I've ever seen, and Brian Michael Bendis and Mark Bagley's Ultimate Spider-Man run (which broke the record of the Stan Lee/Jack Kirby Fantastic Four run) is one of my favorite runs of all time on any comic book series.
26. Pokemon - Shut up, that game was GOOD! So many different characters and powers. I really got into it and enjoyed the show as well. But only for about two years or so then I just stopped following it.
27. Goosebumps - Another book series I absolutely loved. I used to eagerly await every new Goosebumps book and read it cover to cover, sometimes reading it again knowing what the ending was because it was a whole new experience. To this day, I've read more books by R.L. Stine (somewhere in the neighborhood of 50) than any other author.
28. Toonami - I feel like this programming block really got its audience. It's very engaging and fan-friendly. It introduced me to a lot of anime and new programs. Favorite shows include Dragonball Z, Gundam Wing, Naruto, and the Tenchi shows.
29. The Walking Dead - I admit to not reading the comic book series (although I have read the first issue), but the TV show is just so cinematic. I love the fact that aside from two or three characters, anyone can die. And The Talking Dead shows just how strong the fandom is.
30. The Oz Books - There's a reason these books are classics and the Wizard of Oz is so fondly remembered. These ARE American Fairy Tales, and they are spectacular. I've only read the first four books in the Oz series, but I do have the complete collection downloaded to my iPad, and I am eager to work my way through them.
Honorable Mentions - Star Wars, The Chronicles of Narnia, The Hunger Games, Sabrina the Teenage Witch, still new to Doctor Who, and Fairy Tales in general