"Get well soon" is most common. "Get better soon" is possible, but less common. "Feel better soon" is about as equally common as "Get well soon." "Feel well soon*" is grammatical, though awkward/strange

Both are commonly used

Get well soon.

They are both extremely common so you can certainly use either. Personally I prefer not to use them. To me it seems odd to use the imperative in terms of somebody else becoming healthy again. I find Carolina's suggestion to be much more genuine.

First 2 are correct...feel better doesn't sound correct...it's a bit quirky (unusual) but 'I hope you feel better soon' is fine also.