Anything you find enjoyable should be a good speedrun choice. A lot of people here are going to be preferential and biased on what they run so it's best not to ask that.

But if you insist on asking the question, then a counter-question is what platforms do you own, and what genres do you like? PC, PS4, Xbox/Horror, Action, Collectathons, etc, etc.

Speedrunning

Best games to speedrun for beginners!

ResolutionSSB

Fr0gb0y

KaweedFul

Zakkattack7

Pear

landus

Super Mario 64

Super Mario 64 Category Extensions

JidoWu