FIRST!
And get the expansion too. Together they can be had for £5 at play (Supreme commander gold). The original has some problems, but the expansion makes it oh so much better. Play through the original first though if you want the plot, the three factions aoo have seperate campaigns, while in Forged Alliance its the same campaign, you just play through as the different factions, though some characters interact differently with you.
SupCom2 is barely the same game. It was completely stripped back, the maps are tiny compared with the original. The only good thing about it is some of the insane units and buildings. Like the unit cannon. Not as good as the original by a long shot.