A matter of opinion!
I have to agree with Foxmoth about the Robin aircraft and in an ideal world I would use them as trainers but the Factory parts support is just not good enough if you are running a business.
The PA-38 is a very good training aircraft but it is now restricted by the fatugue issues with the main spar and so is now no longer a runner on economic grounds.
So for the two seater it is back to the good old Cessna 152 if you want to keep the price in check.
The four seat trainer issue is a bit more open with both the PA28 & C172 making the grade, I think that Piper is just a little better put together than the Cessna and is likely to be a bit more robust in a training enviroment.