Depends on whether you mean best melee abilities or best base melee.
Ability wise, they are all pretty even, with hunters having best range, warlocks having flame shield and fast recharges, and titans having infinite orb generation
Just based off of base melees, Warlock is best. They have an incredibly long range base melee. And if you're using sunsinger, you have a very small chance to ignite enemies with each hit. Titans is the most damaging. Hunters have it pretty bad with almost no range and pretty low damage (tho I think their melee has the fastest recovery time by default).