I have been to all of them.
If I should choose, I would pick Merida, it is very local, bigger than Tulum and there is always something going on.
Bacalar would me my second choice, very quiet, small, but it has its charme.
Tulum, I didn’t like it much personally, I would go there for a week to explore the cenotes (some of them are very nice) and the ruins.
Buses in my cases were quite on time.