This question is hard to answer without sounding arrogant or full of it. But I promise that in some ways, I'm one of the most humble cats around, LOL. In any case:
General answer: I am pretty driven and motivated, and somewhat of a completionist/perfectionist, so whether it's some tough task in a single-player game, or getting better at a competitive game, I usually achieve it. So far there hasn't really been a VG related challenge that I haven't conquered. (Well, there is ONE, but I'm planning to try it out more in the near future.) Regardless, in any game or genre that I like, I'm competent and play well, but I'm still a hobbyist/amateur -- I don't compete professionally and don't engage in professional speedrunning/glitch-finding/etc.
Best competitive game (retained skills): Super Street Fighter II X/Turbo: Easy game to play but with an incredibly high skill ceiling. I've been playing this game on and off since the late 90s. I know pretty good techniques and know practically every character, with the exception of Blanka and Cammy who are not all that fun to play, and are garbage characters anyway.
Best competitive game - highest high reached: Guilty Gear XX/X2: At one point I was really, really good at this game. I could practically play at a competitive level and would probably have advanced fairly far if I ever bothered entering tournaments, which I never did. I did put a shit ton of hours into this game though, and luckily for me I had some buddies who were themselves amazingly good at the game. So we all pushed each other to get better. But I practically quit cold turkey on the game, and it has been many years since I played it, so I practically lost all of those skills.
Best single player game - it's a tie between (*avatar alert*) Resident Evil 2 (classic) and Super Mario Bros. 3: I can beat those games in practically any way. Speedrun. Self-imposed challenges.
Best genres: Platformers, Survival Horror. I tend to get pretty good at these games.
Worst genres: Shoot-'em-ups ("shmups") - Which sucks because I love the genre, but man, the one time I played Ikaruga, that game humbled me, and humbled me hard. So this is the year that I will get good at shmups!