They all come with drawbacks, real or perceived, and with genuine selling points.
Revere is missing two major tools. Morneau has one pricey year left on his contract, and the concussion history will always cast a shadow over him. Parmelee’s breakout year in Triple A may not have convinced everybody that he’s more than a platoon player.
My guess is that the most likely to be traded is Span. He’s established; he’s a more complete player than Revere; he’s signed to a team-friendly contract.
If the Twins deal Span, Revere moves to center (a better fit than right) and Parmelee can take right field in 2013 and remains Morneau insurance. Should Morneau leave after 2013, Parmelee can then shift to his natural position at first and open right field for Arcia or Hicks.
But as said, it takes two to make a deal. It’s possible that somebody will put a higher value on Revere’s youth and lower contract cost, or on Morneau’s proven power, or on Parmelee’s power and low cost.
For the Twins, it shouldn’t matter. Whichever of the four brings the best pitcher is the one to trade.