I think if you are only going to visit one, then it has to be Inishmor. You can get a taxi (usually there is one at the airfield) into Kilronan for ?5 each way. (They are all minibuses rather than cars).
Lunch at the Pier House is the best I've found.
Inishmann, as much I could get was a toasted ham and cheese sandwich at the local hotel (run down bar, with some rooms would be more like it...hotel
I didn't explor Inisheer at all, but haven't heard of any "establishment" to make it worth while.
Inishmore does make a nice overnight stay on a bank holiday weekend during the summer...especially if the weather is nice...great athmosphere....just book accomodation in advance