WMO/IE 3.0 Masterrace
General: any mouse would do unless its shape makes it uncomfortable to maintain the grip. And it has to have a good laser (IntelliEye technology in my cheap-as-hell WMO works brilliant for me)
Reminder of pro-settings (you can master any settings, just giving those most-probably "correct"):
osu! sensitivity - 1.0
Windows sensitivity - 6/10 (default)
mousefix might help a bit too
change the speed with DPI or osu! resolution
The mice I know which are used by pro mouse players
MS IntelliMouse Explorer 3.0 (past Shizuru, PeaChick (IIRC), rEdo, taNa used to play with Intelli 1.1A and I'm sure I omitted someone)
MS Wheel Mouse Optical - cheaper alternative to the above (me .... ? I remember I saw somebody else...)
Logitech G9 (Shaggon, Niko, ...?)
Those are pretty popular, but some players use miscellaneous mice
SiLvia - Elecom, forgot the model
Doomsday - some random mouse
Gladi - couldn't guess, but looks like some razer (green illumination)
[ Beatrice ] - Logitech G1
the rest I just don't know
^ I've no idea what did I write it for
But you all can see that mouse choice is not so much of a trouble. You don't have to spend 50+$ on mouse for osu!, just buy a decent one. I'm poor as hell so I bought cheap WMO and it was one of the best purchases in my life.
The only exception is when you want to play mouse only. G9 and IE 3.0 are considered the best in this case.