Graham is the most likely, followed by Collins.
Collins should be perfectly fine. She's acceptable on immigration to most republicans, which is the most important issue for GOP primary voters.
The latest polls showed her having something around 30-40% approval among Republicans.
Link? Last Collins poll I saw had her at 65% approval among republicans.
And please don't post a poll from right after ACA repeals. Obviously she would be lower back then. I'm sure she's more popular among republicans after the GOP tax bill.