赶快好起来。（gan kuai hao qi lai） 早日康复。（zao ri kang fu） It's more formal. If your good friend is sick, then you can say:"赶快好起来！" If your friend not very close one is sick, then you can say:"早日康复！"

早日康复。

痊愈 if you are talking to your friend who are sick ,then you can say :"祝你早日痊愈。"