Well, thinking about all of the system I have makes me realize how much I have, but I don't really use much.
Of course my heart goes out to my PC the most, it's what I use every day for several hours, although it's not specifically a gaming rig or whatsoever. Nor do I actually really have such good specs to run top tier games or something.
I've had a PS One, which was I believe my first console and I had a ton of fun on it as a child playing Smash TV with a friend and some good old Crash.
Got a PS2 (slim) later on, quite soon after they got out. I think this was my most used console ever. I was still very young, but, as far as I can remember I've had and played the most games for this. Some highlights would be Rayman 3 Hoodlum Havoc, Gran Turismo 4 (although I'd never play a racing game anymore now), GTA:VC, GTA:SA and a silly spongebob game.
Right now I'm using my DSi again, with a flashcard, and mostly tend to play Dragon Quest IX on it. I've used to play Pokemon on it mostly, and also played a lot of Elite Beat Agents. This was already mostly back in, like, 2007 or something though. I've had a DS Lite before this, but I wasn't exactly careful and so it happened to fall in to water.
I also used to have a PSP, but I ended up bricking it by modding it way too much in the end. I also used my PSP for more than just games, I'd have quite a bit of music on it to listen to and some TV shows like Zack & Cody.
Also I can't forget the Wii. Played the whole Wii Sports hype when we got it, but afterwards used it mostly to play Animal Crossing and to watch anime. I also ended up softmodding it for some homebrew and emulations, but right now it's stored away and basically unused.