Personally, I feel you can make an argument for any of the four to be ranked anywhere from 1 to 4. All have questions, be it health, hitting lower in the lineup, new staffs and leagues, following a career year, etc... Piazza has the highest ceiling so if I were forced to rank them I'd go Piazza IRod Lopez Posada BUT they'd all be ranked within a few spots of each other OVERALL, maybe all in the same round or two?
As for Javy in particular here's my take:
I remeber 2 HRs Javy hit to left center at Camden during an interleague series I attended about 3 years ago. Both were only 2 rows in and wouldn't have been HRs in the majority of ballparks including ATL. So all things being equal I might assume more HR, but all things aren't equal IMO. Typically it's toughest on catchers to change teams, their primary concern (at least early on) is learning the nuances of their own pitchers and the opponents hitters. Their own offense is secondary. Additionally, compared to the other four catchers, Javy is more of a guess hitter and could struggle some while learning the opposing pitchers in the AL.
Here's an unbiased take on the four catchers (I keep an updated spreadsheet that averages any projections I've found...so far I have 10 sources) The composite sheet projects these numbers:
Javy 447AB/ .292AV/ .902OPS/ 30 HR/ 92RBI/ 72R/ 0SB
Piazza 446AB/ .296AV/ .921OPS/ 29HR/ 90RBI/ 74R/ 0SB
Posada 490AB/ .276AV/ .871OPS/ 26HR/ 95RBI/ 79R/ 2SB
Pudge 482AB/ .298AV/ .846OPS/ 19HR/ 80RBI/ 80R/ 8SB