I voted for Tatis. Soto has better rate stats, although his edge in OPS+ isn't huge (175 to 166) and Tatis was just a significantly better masher. Soto had 29 homers and 20 doubles in 654 PA. Tatis had 42 homers and 31 doubles in 546 PA. Tatis was a much better baserunner. Both guys seem to grade out as meh defenders but Tatis plays a far more valuable position. Soto is really buoyed by his tremendous walking total, but I'd rather have the masher. Soto has a slight edge in bWAR (7.0 to 6.6). Soto will probably win it and I don't have a problem with that; Tatis was just the more exciting player and that is probably why I'm siding with him.
Harper was the best slugger in the NL and has a really strong case, but he is just really bad defensively and that hurts him. It's reflected in his good-not-great bWAR of 5.9.