Whatever you're comfortable with lol
Seriously though, its all preference. There's no "best switches for osu", there's only the ones that you think are the best. People find cherry mx blacks, blues, reds, silvers, and silver speed to be the best. Some people find non-MKBs to be the best. Some people dont even play with a keyboard. Just gotta experiment with what you might find more comfortable. If you're looking for the most comfortable switches, buy a hotswappable keypad (or build one yourself), buy some switches (if you built it yourself or if the keypad didnt come with extra switches), and see which ones you like the most. They're not too expensive. If you're looking for most comfortable non-MKB, go to ebay and buy either new or used membrane/scissor-switch keyboards.
Something to note though is that if you plan on buying a keyboard and not a keypad, only use reputable sites. I've seen people recommend keyboards on Banggood, but the one I got from there started breaking down. Some keys were double-tapping, one of my keycaps snapped, and the switch for that keycap got chipped. Wasn't even using it that heavily either. Current MKB I have was from
here and it hasn't started breaking yet. They also got a thing that helps you find the right switch for you, but it doesn't seem any good since its review-based.