I agree so much with what people have posted here already, but just a couple more things.
Organisation and friendliness are key I think. I've only had one TM who was not good, there is no need (in her case) to shout at people or to treat the travellers in her care like children. Enough said.
I do think that wherever possible TM's should accompany travellers back to the airport (realising that of course with regional departures this can be difficult). At Costa de la Luz we were just dumped on the bus going back and left to it. If that is the case JY should have some kind of representation at the airport for going back (unless that was just a one off with this holiday date).
At the risk of making them blush I have 3 favourites.
1. "V" Croatia Istrian Coast. I have been lucky enough to have "V" as my TM twice. She's great. Organisation and friendliness are top notch, and of course she is full of knowledge about her own country. She is also very funny, and integrates with the group very well. I haven't come across anyone in either of the 2 trips I have done with her that doesn't like her.
2. Mark Harbidge (might have spelt it wrong). TM for the first trip out to Slovenia May 2014. Mark was wonderful, it was the first trip for JY, and he was juggling and adding things to the trip to make things more interesting for us. He was constantly on the ball with not brilliant hotels (since changed), and was just excellent
3. Rob Hay. TM on my last trip to Cilento Coast. Such a well organised, interesting guy who worked so hard for us. He was always there to check that everyone was okay without being intrusive.