1. Ninja Gaiden - From the NES games to the Xbox games, this has always been my favorite series. I love everything about them: the ninjas, the difficulty, the combat depth, and the tits. Ninja Gaiden Black set a new standard for action games, and as far as overall package goes, it has yet to be surpassed. It's a perfect game. Then Ninja Gaiden 2 came out, and its combat system blew everything else out of the water. It's completely unrivaled. The change in level design, from a pseudo-open-world to tight, linear stages, with no puzzles to get in the way and little platforming is what action games should be. Unfortunately, the game was rushed out the door and it's flawed in many areas. Most notably the unstable framerate and the completely broken scoring system. But it's still the best action game this generation.
2. Street Fighter - Super Street Fighter 2 Turbo, Street Fighter Alpha 2, Street Fighter 3: 3S, Super Street Fighter 4. Amazing games. The pinnacle of 2D fighting games, in my opinion. I love the near-limitless depth of the series. Even after playing for thousands of hours, there is always something new to learn. Then there is just something basic that makes me love fighting games in general: the fast paced matches. It's a perfect blend of strategy, mind-games, and skill.
3. Virtua Fighter - Basically everything I said about Street Fighter can be applied here. The king of 3D fighters by a landslide.
4. Shin Megami Tensei - Nocturne, DDS, Devil Survivor, Persona, etc. I enjoyed pretty much everything I've played in this series. The stories here are basically the only genuinely good ones in RPGs, the turn-based combat (Nocturne, in particular) is the best there is, the art is fantastic, the music is unmatched. What's not to love?
5. Fire Pro Wrestling - The best wrestling series of all time. Period. They are just absolutely excellent games from top to bottom. Great gameplay systems, UNBELIEVABLE customization, MASSIVE roster. I'd kill for a Fire Pro HD with GGPO-quality netcode. I'd trade nearly every game I own for it.
6. Devil May Cry - A different breed of action game, where the focus is not on vicious enemies, but on stylish combos. A perfect alternative to Ninja Gaiden.
7. Demon's Souls - This game stands head-and-shoulders above nearly everything else this generation. Everything about it is basically in a league of its own: from the excellent combat and controls, the art, the music, the level design, the one-of-a-kind online play, the endless replay value, and maybe my favorite thing about it: the complete lack of hand-holding. The further we get into this generation, the wimpier developers have become. Demon's Souls is not afraid to punish you for playing stupidly. Die? Replay the entire damn level. Die again? Say goodbye to all the souls you've gathered. I'm glad to see there's still an HD developer with balls.
8. Ouendan / Elite Beat Agents - It took me a while to warm up to this series, but once I did they quickly became my favorite music games of all time. I'm not sure what to add here since everything about it is awesome. I guess my only complaint with the series is I wish they would let you play the higher difficulties from the start, since the regular ones are a snoozefest.
9. Pokemon - The competitive aspect of this series is what really draws me in. Basically, the game really begins once you finish the 30 hour long tutorial known as the story. From there, it's time to plan which team you want to build, go and find the pokemon you want, breed them to get a good nature and IVs (this part sucks), and finally train them against specific enemies for optimum stat growth. Then it's time for multiplayer battling, and this is where the game really shines. With nearly 500 Pokemon, the amount of strategies and tactics available to the player is staggering.
10. Dodonpachi - I wanted to choose one shooter for this list, but I wasn't sure which one. So I'll have to go with the Donpachi series, even though I haven't played the latest entries (DDP DOJ BLEX and DDP DFK). I can't say I like the chaining system either, but I can overlook that for everything else the games offer. Namely, the great bullet patterns, strict challenge, great music, and good art design. One of my goals is to 1CC Dai Ou Jou one day, but goddamn that game is hard.
11. Steel Battalion - I love the complexity of this series. The big beefy controller, the awesome mechs (er, "Vertical Tanks"), and with SB: LoC, the persistent online war. Playing Line of Contact online in big 10 player matches on a giant map, where every death MATTERED is a gaming experience I will never forget.