This topic could probably go elsewhere but...
I suggest against the following: To Lands Unknown, A New Order, Invasion from the Unknown, Afer the Storm, Legend of the Invicibles, Bad Moon & Trinity. And there is a reason for this.
Those are some of the biggest, best, most interesting designed UMC campaigns. And that's the problem. Since you admitted to being new, getting a handle on the tactics and AI thought process is pretty important before stepping into these campaigns. Otherwise these can just put a beatdown on you, leaving you frustrated and giving up before you really see what's in store. Espically because they either have new armies and/or new systems that are not in the base game, making things a lot more confusing and frustrating at times. And three of those(Legend of the Invincibles, Invasion, Bad Moon) Are pretty long too, taking a good chunk of your time. More so if you want to play the sequels(After the Storm and Trinity) where as Invicibles is just bloody long.
Great these campaigns are, newbie friendly is not something I would say about them. Go ahead and try them, you might have a blast and be far more skilled than I give you credit, but understand these are not the campaigns I'd pick out for a new player. The others I can't give an opinion as I haven't played them.
For more 'friendlier' Campaigns, I'd suggest Brave Wings, Across the Ocean, and Oath of Allegiance. Pretty well put together if I say so myself. I also might be overstepping by this entire post but "Best UMC Campaigns" doesn't necessarily mean "Best UMC for New Players". Just my two cents.