Depends on what you are looking for. Detwiller probably gets more wins playing for a better Washington team. Fernandez will rack up the k's, but supporting cast will limit his wins.
I like Estrada. His e.r.a. is a bit high (he does give up the long ball), but his strikeout rate is very nice. With this guy, you just have to know what you expect. Expect an e.r.a. around 4.00, with a WHiP of 1.3 (maybe a bit less), and a ton of strikeouts. I expect Estrada could push fifteen wins by season's end (it would help if Aramis and Corey hurry back--and if the bullpen gets it's act together).
I wouldn't even think about moving Masterson now (at least for a f.a. add--if you get a good trade offer, then maybe consider). Masterson won't be THIS good all season, but he's young enough (28) to take another step forward. Pitchers tend to mature a little later than hitters. I tend to like pitchers at Masterson's age (other age 28 pitchers: Jeff Samardzija, Gio Gonzalez, Kris Medlen).