Best Starting Faction?

The glorious United Empire, because it is led by none other than Zelevas, the most handsome emperor in the whole Galaxy.

I wouldn't lock it to just one.  There is no best faction in this game, just like there's no worst faction.  They're all just very unique, which I think is this game's greatest strength.  And while I do have my favorite factions like the Vodyani, Riftborn and Vaulters.  I wouldn't consider any of those the best starters due to how unique their play styles are.  If you've never played 4X games before, then sure, your learning curve with one faction should be the same as any other.  Except for maybe the Riftborn and the Hissho.  I think those two factions are probably the most punishing factions to learn to play when you're new to the game, due to the riftborn requiring you to know that you need to build their pops yourself.  And the Hissho due to the Keii system.  The Vodyani was close to being in that list for being hardest to learn with, but they're just barely easy enough to figure out and play competently to not deserve that designation.

But if you have played many or even some 4X games like Civilization before, then I would say either the Sophons, United Empire, or the Lumeris would be the best factions to play for your first games.  Their play styles are the closest to traditional factions play styles in other games.  With the one notable exception of the Lumeris buying their colonies rather than needing colony ships.  Plus each one specializes in the more familiar aspects of traditional 4X games with the Sophons specializing in Science, the Lumeris in Economy, and the United Empire in Production/Influence.  

They are also the most straight forward factions to play, requiring the least number of "additional" gameplay mechanics to be played competently.  Such as the population screen for gene splicing Horatio.  If you don't know it's there and what it's for, because it's your first time playing, then you can't play Horatio the way they are meant to be played.  Which is the only reason Horatio didn't make my list of best factions for "first time" players.

All the factions are great, but the Sophons, United Empire and Lumeris would be my top picks for beginners.

I would vote Vaulters, cause they skip the outpost/colony ship production mechanic and thus are simpler. Ofc that is with the DLC, thus is not a suitable starter.

1) I would say United Empire. They do not have any hard unique mechanics around which gameplay revolves.
2) Unfallen are amazing, they are slow pace, they just expand and grow, mostly without battles. Good to learn what and how. Although their way of colonizing is different. Also recommend.
3) Cravers are micromanage demanding so you need to understand basics of the game and then dig into their stuff.
4) Riftborn are basicaly reverse to other races.
5) Vodyani and Hissho should be touched untill you got some experience. They have a unique stuff around which you have to play.
6) Can't say anything about Vaulters do not play them. They are boring for me.

TL:DR: PLAY UNITED EMPIRE or UNFALLEN. Learn basics and then switch factions to find the one you like the most or the gameplay of one you like the most.