I would go:
Lester - He's been the most consistent starter IMO. He's averaging 6.1 IP per start, leaving only 8 outs for the bullpen. He's got a 2.45 ERA in his last 5 starts, and has 21 K's and only 6 BB's. He started out rough, but he's the ace of the team.
Beckett - After that whole lat strain followed by the golfing incident/Cleveland game, he's settled down and been as good as anyone in baseball. He's had 2 horrid starts, but outside of those, he's been excellent. He allowed 14 ER in 2 bad starts (25.20 ERA), and 10 ER in his other 6 (2.03 ERA). That equates to allowing 58.33% of his total ER in 25% of his total starts. He looks like he's settled in nicely, and I think we'll see two very well pitched games this weekend vs TB.
Doubront - Doesn't get deep into games, but doesn't allow a lot of runs either. His ERA of 3.96 is 2nd among Red Sox starters to Lester (3.95). He's turning into a solid starter, just needs to get deeper into games and focus after 2 outs because right now, he loses focus after he gets the first 2 outs and then ends up with a 2 out walk.
Bard - Really needs to get his control and some velocity back. He looks like he's throwing the ball sideways. It's very strange. He's not on top of the ball like he used to be, his fingers appear to be on the side of the ball, and his velocity has dropped a lot. I expected a velocity drop in the 1.5-2.5mph range, but 4 mph off his FB is crazy. And he has lost a lot of control. I'm not sure this experiment is going to work.
Aaron Cook - Yup. Buch doesn't make my list. Cook had a numb leg after the 2nd inning of his last outing. His sinkerball could workout well as a 5th man, go deep into games and make it a quick game. Works fast, keeps the defense on its toes without lulling them to sleep. Buchholz needs to head back to AAA to figure his s*** out. Until then, Aaron Cook (when he is done rehabbing), should get the nod as the 5th man.