The shiv does not learn does not feel and will sacrifice itself to save you.
The support class is the best all arounder.
Have more people than you think you will need
Try to avoid having only rookies on missions.
They system buffs enemies if you research weapons and armor so be ready to build it as soon as you've researched
Heavies cannot hit the broad side of the barn for large parts of the game.
Build the gotlieb device. Gain psychic powers. Destroy enemies with the mind war.
Beware of crysalids, if they kill they multiply
Also snipers get broke as hell once you have squad sight.
@zebratron2084
In that case. Cyborg! Trust Shen! But keep some normals around to farm for psychers.
Oh also beware exalt!