O classic difficulty I like to have:
2 snipers: both with squad sight. They are just too powerfull, they are my main killers.Plus having 2 disabling shot makes stunning enemies a lot safer.
2 heavies:1 goes holo targeting/shredder rocket, the other goes bullet swarm/suppression. Heavies are juste incredibly usefull in all situation. Holo targeting helps everyone, suppresssion can save a life, multiple rockets for your big "I'm dead if those two aliens dont die immediatly" moments or just to blow up alien cover or to blow up walls to give a shot to your snipers who is all the way in the back. Way too usefull all around to only have 1.
1 assault and 1 medic