Resident Evil 2 by far. Not the entire game though, Leon A is my favorite (I think this is the case with alot of people who run RE2.) Leon A just happens to be the most optimized scenario and also the fastest, but even if it wasn't I'd still run Leon A over the other scenarios bc it's the one I grew up playing the most lol. Leon from RE2 is one of my fave video game characters ever, it's almost like he's a different character once he changed in RE4. :/
Probably Mario Party 9, I just have a lot of fun with the Boss Rush category. I could imagine that the other Mario Parties are fun too, but I didn't run them.
smw and alttp were fun race games when I did them. plenty of categories. also pretending to play pokemon games fast was always a kick
There's so many games that I love speedrunning, I couldn't choose just one game, but game series, it would either have to be the Sly Cooper series, Jak and Daxter series, or The Call of Duty Zombies series.
Whatever strikes my fancy when I look at my collection really. At the moment though I'm currently trying (and failing) to run Resident Evil REmake.