S.T.A.R.S. Wesker's magnum is (slightly) stronger than Chris's in Mercs/Versus. While a fully upgraded M29 is stronger, the Mercs/Versus M29 isn't at its max. There's mostly no difference when killing players, but there is a 1-2 shot difference when killing minibosses. As for the M500, Midnight Wesker's magnum is great for minibosses, but when it comes to killing players, its rate of fire and recovery time are too slow to be as effective as the S.T.A.R.S. characters' respective magnums.
With Tribal Sheva, you need direct contact (at least from my experience) to freeze enemies, and two headshots to down a full health enemy, and this limits the range that you can effectively use the combo to quickly kill players. You can kill them in three shots, but the time needed to do so let's the other player recover. Still a great character, though, I'd probably rank her third.
Chris S.T.A.R.S. is the best at getting quick and certain player kills. The electric (or nitrogren/flame) round -> 2 magnum shots to the gut is certain dying state and unavoidable. Electric rounds also don't need direct contact to connect.