I think if I were to pick a game today to invest time into learning to speedrun, it'd probably be Thief Gold, Thief 2, or both.
I love those games, and speedruns of them are all about knowing the maps, how the AI works, and pretty precise movement rather than lots of glitches and a ton of rng. Plus I like the thought of drawing more attention to them because they're great
I’ve thought about speedrunning before and what games I would do. There’s just one problem
I suck at video games.
I guess if I were to speedrun any game it would probably be the N64 Zeldas, seeing as those are the games that I am at the very least competent at. I also tend to use a lot of speedrunning techniques when playing those games casually like back walking and such.
Spirit Tracks is another contender, but only because close to nobody runs that game, so it would be very easy to get WR on.
I'm certainly not a speed runner, but pretending I were as you say, I think I'd go with Super Mario World. It's the game I've played through the most times and I still finish it at least once per year (using the star road of course).
The game that I know the best and can complete in around 8.5-9hrs depending on certain factors, without knowing any glitches or exploits, is Star Fox Adventures on the GCN. It’s definitely the game that I’ve played the most over the years since it was one of the first I ever owned.
Probably the Resident Evil 2 remake. It's the only game I've ever come close to speedrunning (gotta finish a campaign in less than two hours and with three or fewer saves) to unlock that glorious S+ rank. Since I'm not actually good at video games, it was incredibly stressful. The plus side is being able to take an infinite ammo rocket launcher or minigun into subsequent playthroughs.