I've found looking for someone who matches your vibe and shared interests works best. There are some girls on the sites who I like physically, but I can tell by their profile (or their decision to not even
bother filling out a profile) that we aren't going to be a match. You can usually sus this out within a few exchanges on the site.
Obviously, everyone is looking for something different. For me, if the girl responds with a positive attitude, questions for me, and at least reasonable punctuation and grammar that is a huge green flag.