While this doesn't take into account how fast you can kill, I ranked it by lowest to highest exp overall to see where it may make sense to drop a merc sometimes if you can kill without one for example.

4 people - Green - 8
5 or 6 people - Green - 8
Trio - Green - 9
5 or 6 people - Light blue - 11
4 people - Light blue - 12
Duo - Green - 12
Trio - Light blue - 14
Duo - Light blue - 18
Solo - Green - 20

5 or 6 people - Dark blue - 26
4 people - Dark blue - 29
Solo - Light blue - 30
Trio - Dark blue - 33

5 or 6 people - White - 41
Duo - Dark blue - 42
5 or 6 people - Yellow - 44
4 people - White - 45
4 people - Yellow - 48
5 or 6 people - Red - 51
Trio - White - 53
4 people - Red - 57
Trio - Yellow - 57

Trio - Red - 66
Duo - White - 66
Solo - Dark blue - 71
Duo - Yellow - 72

Duo - Red - 84

Solo - White - 111
Solo - Yellow - 120
Solo - Red - 131