All castles in Heroes 3.

If you compare the Rampart and the Tower, much depends on whether you are playing with 200% resources or less. If all your resources are at zero, the Rampart has an advantage: High Elves are built faster, and they are good warriors. And it's much more pleasant to use Centaurs as cannon fodder later on. Also, the first level of the Conflux is good in the early stages of the game.