#2 Cain Velasquez - Currently a free agent
The former UFC Heavyweight Champion has been in talks with all of the big wrestling promotions in the world from WWE, to AEW, to New Japan Pro-Wrestling. He's only wrestled for AAA so far, but those have been per-performance deals.
Velasquez's decision comes down to many factors. If he wants to do strong style, then he'll go to NJPW. If he wants to continue the relationship he's forged with AAA and Cody (they teamed together at TripleMania this year), then he'll go to AEW and continue with AAA.
If he wants to make the biggest impact and likely have the highest profile matches, however, then the only place that would make sense is WWE. He and Brock Lesnar have history in the UFC, going back to when Velasquez took the Heavyweight title from Lesnar. It indirectly led Lesnar away from MMA and back to WWE. If WWE wanted to play on this, it would be easy to write.
What stage is bigger than WrestleMania? Cain Velasquez is 37 and his window - unless he's a part-time performer like Lesnar - is closing. He's a big enough name to wrestling fans and that would mean he wouldn't debut to crickets like Tito Ortiz in Impact.
Verdict: Will likely sign with WWE for the Brock Lesnar program. No other promotion can offer him the huge exposure that has been given to both Ronda Rousey and Lesnar.