At 75 for merits it would either have to be.

3xWAR/NIN, RDM/WHM, BRD/WHM, BRD/NIN

or

4xWAR/NIN, RDM/WHM, BRD/NIN


For most normal (non-burn) merit builds I prefer a NIN tank with a THF to help plan hate. I can't really say one PT set up because they are basically all the same thing as long as you have the right people filling the roles.

Tank, 2xDD (who can SC well together), BLM, WHM, RDM (or BRD or COR)

or

Tank, 3xDD, WHM, RDM (or BRD or COR)

I honestly think that in a PT of people that know what they are doing that the PT with BLM is more efficient, but you can't expect that all the time.

I'd rather be in an outlandishly odd PT of people that know what they are doing than a perfect set up with noobs. Most often the case is that if you have a group of people that know what they are doing the good set up will follow naturally.