Wieters (Most upside/but riskiest) Sandoval (Safest bet, Least likely to hurt your team) Iannetta (Power is back?) Soto (Disappointing this year but we know he has potential.)
I'd say it's very close and really, any one of these guys can rise to the top of the list in a couple of weeks. I'm lowest on Soto, but there are many signs that he should be gaining some life soon.
For now, I'd go: Wieters (Bad start, but he's beginning to heat up and the upside is just too great) Iannetta (I'm a big fan. The power is for real; it's not just the Coors affect) Sandoval (Solid, yet unspectacular. If what you want is a safe bet, he's your guy. If you want the high-upside move, go with one of the above.) Soto (His year last year could easily be matched by Wieters and/or Iannetta, but something tells me, Soto won't have that same level of production from here on out.)
Wieters- Huge upside and will undoubtedly be a good catcher Sandoval- Just a decent catcher not too much to say Soto- I still feel like he will turn it around this year. Ianetta- Closely follows Soto