Hmm... it's a hard question.
I don't know what my favorite games would be since what I've been playing the last year is different from before and so on.
Years and years ago, I'd have said Pokemon, but that was like 18 years ago. It has been like 15 years since Pokemon has been good, anyway.
Right now I love Beatmania IIDX and other rhythm games, but I haven't played as much as I'd like lately (a lot is just the fact that handling audio sync on PC is a massive hassle so the games just never feel quite right... at least, if they don't offer exclusive audio mode which fixes everything and I don't know why there are any PC rhythm games that don't offer it). If I can drive out to the arcade, I'm happy, but that's a long trip to go play, I can't do it every day.
IIDX arcade data has a terrible habit of crashing on PC too...
I should just go buy a PS2 IIDX controller, or even hack one up myself, solving the issue entirely.
I used to play a fucking bunch of Groove Coaster and got pretty good, but haven't in months. I can do like 2-3 songs in Project Diva Megamix+ on Extreme, but I haven't played much lately, been trying to get back into it.
Years ago, I used to be big on shmups and that was definitely my most played genre... but there was a point when I realized I was just bad at them and wasn't really getting better, so I stopped bothering.
A few years ago I was very into fighting games, but now I'm getting older and slower and I wasn't amazing in the first place, very middle of the road. I haven't played any fighting games in months now.
I like arcade racing games, but most of the genre is like 25+ years old. I fucking love the PS1 Ridge Racer series and the PSP Ridge Racer 2, and if I still had a PS3, I'd play RR7 again (shit man, I'd emulate but IIRC it still has issues in RPCS3)... but I haven't played any of those in months either.
tl;dr: shit's hard man
but I guess right now it is rhythm games