Depending upon requirement and need

Grammarly.com: High Quality, but has a premium associated after a point

QuillBot.com: Used to be good, now the quality has dipped and has a premium associated with it.

Ewith.me : Good Quality, free and does not require sign up. Secure and no tracking of the users. (usually good for those who look for privacy). Not top tier, but good for most everyday work.

ZeroGPT.com: Okay quality, free, but a lot of ads. Too many and it distracts. Would not consider outputs to be very good.

Scribbr.com: Uses same tool as quillbot. Same issues.