I believe he's the best by defensive metrics, so far. Not sure if he is the best CF defensively by projections.
I think Cards consider Carlson to be their CF though.
If you look at "sprint speed" and "outfielder jump" on statcast it paints the picture:
SS percentile on left, OJ percentile on right
Carlson = 71st, 28th
O'Neill = 98th, 53rd
Nootbaar = 74th, 50th
Tapia = 72nd, 10th
Springer = 76th, 57th
Simply put, O'Neill has a better full-out speed than the other two and he gets decent jumps. Carlson isn't as fast and struggles off the bat.