4 githyanki fighters - invasion of the killer frogs
Battletoads.
The classes that work best for a single-class party are the ones that have a diversity of build options. Probably the best option is Druid, because they can melee, heal, and cast spells. Clerics with different subclasses would probably also work out well.
But with how reliant this game is on doing huge amounts of damage to make it through combat, you could easily do 4 Fighters, Barbarians, or Paladins (one of each Oath, maybe?).