In no particular order :
Pokemon
Just like you, I laughed at the idea of the game. I dismissed it as "Just for kids", with way too much merchandising attached to it. But, things changed one fateful day. My nephew had gotten a copy of Pokemon Yellow, but had managed to get stuck. My sister knew I played a lot of videogames, and asked me if I could help him out. It was a Sunday afternoon, and I didn't really have any plans, so I figured "Why not? It'll probably take me 5 minutes."
I was mistaken.
Turns out that my nephew had traded his starter Pikachu for a Gyarados with some kid at school, in order to beat the first gym. Which was all well and good, except that the Gyarados was level 20, and thus wouldn't listen to a trainer without badges. Somehow, he had managed to defeat Brock, but now had to face Mt. Moon. This was difficult with a Pokemon that simply refused to do as it was told, and to make things worse he hadn't bothered catching any other Pokemon that would. So, I quickly realised that I needed to rectify that, and set about catching a few of my own.
It took me 4 hours to get through Mt. Moon. By which time I was hooked, and the very next day I went out and got my own copy.
What got me, and what still does it for me, is the sheer depth of the games. Underneath the cute exterior beats the heart of a STAT-OBSESSED UBERNERD! It is dizzying, confusing, and extremely satisfying once it all falls into place. I have spent well over 200 hours on each generation since then, because once it gets under your skin it can hold on like nothing else.
Street Fighter
As a somewhat competitive individual, nothing pleases me more than a game that allows me to play against my friends. (Except a game that lets me play WITH them!) Street Fighter II encapsulated everything that should be included in a competitive game, and set the template that all fighters since have had to follow. There are other fighters, but none that have the same kind of balance. Basically, the best player usually wins.
It is about more than the combat, though. Virtua Fighter is, if anything, even more balanced and rewards skill even more, but it just isn't as pleasing to play. The characters in VF are fairly standard, whereas Capcom's gallery are quite perfect.
Mario Kart
The SNES original took me totally by surprise. I was a fan of the Super Mario games, of course, but the thought of those guys racing each other in Go-karts? Yeah, not immediately obvious. However, from the very first play of the very first game, it revealed itself to be absolute bucketloads of fun. Having weapons in the game turned it from a potentially stale race into an exciting and hilarious contest. But, the real genius was the inclusion of the hop/slide mechanic for cornering. This is what gave the title legs, allowing it to expand beyond one play-through by giving scope for real concerted time trial racing. Shaving micro-seconds off of a time is a wonderful feeling, and being able to maximise the gain by skillfully manipulating my kart is so much more fun than reaching a set maximum speed and sticking to the best racing line possible.
Don't get me wrong, Gran Turismo and Forza are fantastic games. But, compared to the invention and insanity in Mario Kart, they are more than a little sterile.
Phantasy Star Online
I got to play a fairly simplistic game, but I did it with friends. I did it for rewards that had no purpose other than making the game itself even more fun to play. I gt to customise my character, and do stupid stuff with visual chat bubbles for pure comedy effect. Nirvana? Pretty damn close. Sometimes, I don't want too much story to get in the way of just laying the smack down on a load of monsters. PSO delivered everything, and remains my favourite dungeon crawler of all time.
