Here's my opinion on which is best:
1. Melee. The devs love it more than the other 3, it has a lot of cool weapons, and unlike other ones, it has a lot of good accessories.
2. Ranger. It's halfway decent, but I dislike the overreliance on specialized ammo.
3. Summoner. It's actually pretty good, but whip stacking and minion AIs make it too complex. Also, I hate Pumpkin Moon.
4. Mage. It can be somewhat powerful, but it bothers me to have mana sickness all the time, and the accessories tend to be hard to get.