As a ROBLOX speedrunner, I would recommend "Adventure Forward 2: Points of Conflict". It's a pretty fun game in my opinion and there are many glitches (wallhops, lag clips, etc.) you can use.

But it's also pretty difficult so your first run may take at least an hour or two.

there are many games and it's up to you to choose. here's all of them, https://www.speedrun.com/roblox#platform=&orderby=mostactive&unofficial=on

Any Roblox Games to Speed Run?