I'll start off with the team that I know best, the Detroit Tigers. The Tigers are currently leading the Central and only have a few small moves that I would make.
Starting with the position players, the outfield is completely fine with two ROTY candidates in Brennan Boesch and Austin Jackson in Left and Center, respectively, and Magglio Ordonez, who's had a great bounce-back season from last years poor performance and has done very well in the #3 spot. Johnny Damon has also started to heat up and with him normally playing DH I think all of those spots are fine. Miguel Cabrera is at first and is, in my opinion, the current AL MVP. Alex Avila and Gerald Laird aren't wonders on the offense but their defense more the makes up for it. Carlos Guillen has done well, average wise, since making the move to second base and his numbers are respectable for a number 6 hitter.
The left side of the infield is where the Tigers problems, hitting, happens. 3B Brandon Inge is receding from his all-star season with low power numbers, but his defense always is top notch so he will stay. Shortstop is certainly the weakest position, offensively, for the Tigers. Ramon Santiago and Danny Worth have been splitting time, but both are liabilities. I've read that Stephen Drew of the Arizona Diamondbacks is available and is locked up for more then one year, so the Tigers should go after him. His power numbers aren't great, he only has 4 HR's, but he would be an improvement at shortstop by tenfold. The Diamondbacks would more then likely be looking for players that are almost ready so I wouldn't be afraid to give up Wilkin Ramirez, who's an OF and will never probably get a fair chance in Detroit, and one or two pitching prospects, such as Fu Te Ni and Alfredo Figaro, who have some big league experience.
As for the pitching, the top of the staff, with Justin Verlander, Max Scherzer, and Jeremy Bonderman is very solid. Armando Gallaraga hasn't been terrible since his perfect game, but he hasn't been all that great, either. Rick Porcello has spent the past few weeks in the minor leagues but I'm thinking he'll be able to turn it around and pitch like he did last year and be sent up soon, which would be a plus since it would be like adding a new name. If you put Porcello at 4 and Gallaraga at 5 you'd have a back end that, if on their game, will give you a win. How often that is though, is the question.
The bullpen has taken some hits, though, for the Tigers. Joel Zumaya had been having a great season as the set-up man before injuring his elbow, making him out for the season. Zach Miner and Bobby Seay were both out from the start of the season, so that didn't help either. Our closer Jose Valverde has been the best in baseball this season, but other then that who knows. Phil Coke seems to have filled the set-up man role, at least for now, but seems better suited as a left-handed specialist. We're hoping Ryan Perry's time in the minors has straightened him up, as he was having a great start to the season before a few bad outings. Daniel Schlereth and Robbie Weinhardt have both just made their debuts, but their numbers in the minors were quite impressive. Then you have Eddie Bonine and Brad Thomas as the long relief men, who have done an adequate job so far.
So, to wrap it up, while for the short term the bullpen seems fine, the Tigers, aside from Valverde and Coke, probably need some late inning help. A set-up man, most preferrably. Perry could turn it around and be that guy, but atm I don't trust him, so I would try to get someone like Octavio Dotel, who has had experience pitching in the late innings. As long as he's on the market, that is.
Their main priorities should be:
Late Relief Pitcher
Short Stop
#4 or #5 Starter
Catcher
In that order.