Hey Guys,
I just wanted to add to this forum post. From my research and now using the One and watching reviews and such, the One is the best performing tablet that you can get at the moment. I now own a One and can tell it already made me a better player after using the Gaomon S620. As NikuCzeko said, the friction is really bad meaning you will need a tablet cover, but personally, I would always recommend a tablet cover just for the protection of your pen nibs. The next tablet I want to talk about is the 4100p by Wacom. This will probably be another one you hear recommended. The 4100p is just an upgraded One with a better pen. The trade-off is $20 USD, which to me does not make it worth it for an upgrade. Although, if you plan to do some graphic design, then it 100% is the tablet of choice for you.
To wrap this up neatly, the One by Wacom is the best tablet for osu!. If you'd like to do graphic design and play osu!, then the best tablet is the 4100p by Wacom. You will need a tablet cover for both as the surface is very rough and go through pen nibs very fast. To add, the best tablet imo for your money if you're looking to save is the Gaomon S620. Again, hopefully this helps!