We have sailed:
NCL - twice - I think I have the fondest memory of the 'Sky' and 'Sun' because the former was our first cruise and the latter was our Alaskan cruise which, so far, was my favorite and I really have no complaints about those ships.
RCL - twice - 'Enchantment' and 'Adventure'. I think I liked the size of Enchantment the best over Adventure - although a gorgeous ship, I think I'd rather sail on a more moderate-sized ship.
Princess - twice - 'Golden' and 'Star' which are essentially the same ship (same class) - they were my favorite. The size and layout of the cabins were good. The service was excellent and the shows were the best I'd seen at sea.