Good to see mostly level headed responses instead of bragging. I started gaming in the mid-80s, at the time most of it was platform/puzzles/point and click adventures. I try pretty much everything, I mostly avoided sports games (no interest) and shooters after Doom 2 (burned out on Wolfenstein and the 2 Dooms). Favourite genres are usually some type of RPG, and yeah I played plenty of Diablo-likes.
I'd say a lot of it is experience, as well as knowing how AI works (I'm also a programmer). For example, in some games you know when you present a threat this way, the AI will respond that way. This is why I find some games hard to enjoy, because I can see the innards working (or not working, as the case may be).
Practice is part of it but most of the time it's more of spotting patterns. It gets disillusioning at times but that's just how things go - move on to another game. For example early racing games: remember how people like to use them as example of bad AI (rubberbanding)? You'd sweat for 5 minutes using every inch of skill to remain no#1, but the instant you make one mistake the entire pack of AI cars zooms past you and it takes another 5 grueling minutes to regain your lead. Spotting bad AI like that quickly sours me off the game, even if there are similar exploits to keep yourself winning.
I love space sims and strategy games, the more sedate ones like Civ rather than the MOBA type nowadays. Can't really say I'm all that good at them, for example I don't really care for min-max "1st action do X, 2nd action place building Y, 3rd action queue production for units Z", cookie-cutter efficiency is boring as shit, I'll leave that to the multiplayer weenies; I play games to have fun, besides figuring out the AI and how to improve them (I also mod games).
If I had to say which genres I'm good at, it's kinda hard - I don't suck at most of them, but I wouldn't care to say I'm particularly competitive either. I dislike multiplayer (unless it's IRL e.g. LAN gaming, because friends). Games that invite a lot of preplanning are nice though especially RPGs which lend themselves to a lot of number crunching. Training is kind of a must, besides planning there's also luck, and how to factor that in - I hate it when people blame the RNG, factoring the RNG in with everything else IS part of skill. Never played pro; though there was that first year in college where I made money inviting passers-by to our campus dorm to play Street Fighter. Just pocket change for takeout, nothing serious. I won most of the time and this was a campus of 100k and plenty of international students so I like to think I wasn't just a village champ, lol. Still, I acknowledge that was mostly random people, not like an actual tournament, more like challenging random people in a pub to a game of darts. Hey, it was easy money, and a fun way to meet people.