I use my middle finger, but it hurts my hand like I'm gonna get the RSI stuff if I do fast BPMs for a long time. From what I observe top players mostly use K2 (index?) as main (cookiezi angelsim). Could it be to avoid the RSI stuff? or we just need to get used to this stuff? Non-mechanical keys here.
*RSI = repetitive strain injury
**EDIT
A lot has changed since this post was made. I changed my keyboard, and after just a month! your hand can get used to osu. Well the pain was feelin real tho, maybe you should stop and try playing again tommorow when it happens. Now I don't experience pain after long streams etc. my stream is slow though