dnd 5e – What class/multiclass options have the most damage output per round?

I used to DM a group of players who liked to heavily optimize their characters (and the group as a whole) to take as little and deal as much damage as possible (seriously, these guys were crazy). It got to the point that I was routinely throwing encounters at them that would be considered “deadly” for a group of their level. Unfortunately, some of the members of that group moved away at around 6th level, so I never got to see how it carried on to later levels.

I recognize that this question is very broad, so to clarify and narrow it down a bit:

What are the most optimizable class/multiclass options for damage output per round in a single encounter against a small group of high HP enemies at levels 5-6, assuming the enemies have no damage resistances and the character has just finished a long rest (i.e., all abilities are available)?

To make this less daunting, let’s say that at maximum 2 classes may be used, and the second class needs to be chosen at level 2, using only the classes from the Players Handbook.

Also, I’d like to state that I’m not looking for survivability here, so things like HP and AC are irrelevant. It’s fine to include magic items, as long as it would be reasonable for the character to have the item at level 5-6.