I'll go reverse-chronological for recollection sake...
SJDS: Nadiya, Kelley for me...I was shocked to learn that Nadiya was the more "trusting" of the twinnies in a few post-finale interviews she and Natalie did together; on amazing race, I always saw Nadiya as the in-your-face one, with Natalie tagging along quietly

- nevertheless, if Nadiya could play more conservatively, she'd be a force; Kelley was a superfan I wanted to see shine more, damn Misch.
Cagayan: Brice, kid could have been hilarious TV for us come the merge, causing drama and flipping people every which way, huge social threat potential; also Garrett could have done some wheeling/dealing
One World: Poor Courtney Moon, give her a early boot season slot! But if I had to pick someone impressive, Colton, he almost ran away with pre-merge and then he just decided to quit

Samoa: LIZ was amazinggg, end.
Redemption: Stephanie could have caused mayhem and really played hard out the gate
Micronesia: Kathy..hilarious..only quitter I ever loved, she had such a great spirit for just being on the island initially (before it broke her) and getting to know everyone
I'll think more on this, would love to hear what other people's pre-merge favs are and how they did well.