Hatton asked about the best aircraft to start a group with.
is true, and I agree that the direct answer to this question, in the context of this discussion is the 172.
However,
and quite why people describe them [150/152] as good trainers is beyond me.
My opinion of why they are good trainers, as I also think a Tiger Moth is a good trainer, is that they are not stellar in performance nor handling, but are unusually safe. A good trainer is not the aircraft that can do everything well, that's the good working aircraft. If a pilot can manage an anemic aircraft well, they are well positioned for transition to other types.
For a few pilots I have cross trained, who I know were very experienced on more capable types, an element of my training (which took a lot of time and demonstration) has had to be that this aircraft will not simply out climb something straight ahead, you'd better plan. It will not just haul a load off a short runway, you will have to refer to performance information, and apply a suitable technique.
The foregoing may be small "deltas" for long paved runway operations, but when I have a Cessna 210 pilot whom I'm now training in a 172 on floats, it can be critical. A good training aircraft will not be a ball of highly maneuverable fire in the hands of a new pilot, for the same reason we don't normally train drivers in Ferraris. The trainer must demand good technique, planning and patience.
For the few times I have had to train a pilot in a new type which was a real performer, I have reverted to artificially reducing the capability of the aircraft. When finishing the training of a fellow once in his new (to him) Lake Renegade, I required all takeoffs to be at 60% power maximum. He reflected later that my requiring that of him was the single most formative element of the training, and had been a great benefit over the training of the other instructor who had allowed him to use full power.
There have been times I have said to another pilot "well, that worked, but it should not have". Meaning that the capability of the aircraft had masked their poor technique. A "good trainer" will not mask poor technique with its own performance or handling. But, on the other hand, it will not be so difficult to fly, that it frustrates the student, or is dangerous.
It is for these reasons that one must really understand the intended role of an aircraft before choosing it for the job....