Which one should I get?
Command & Conquer 3 + Kane's Wrath. The campaign for vanilla CnC3 is amazing while Kane's Wrath introduces a nice new load of subfactions/units to change your way of play.
Red Alert 3/Uprising is probably only worth $2.49 together, feel free to purchase it if you're into stuffty plots, tower defense maps and cleavage. It's barely okay. Not too great.
CnC4 is just an absolute cancerous atrocity. Do not purchase it. Do not even consider recommending it.