I'm not really big on the warfare side of this game (im new to 4x's and struggle with this side always though improving) so i was wondering what factions have the best non-combat way of winning? 

1. Umbral Choir. They start with cloaking and a single hidden System, meaning they don't want to fight in the first place. A lot of other factions are good at pacificism, but you can't fight the umbral choir if you don't know where they are in the first place. Even if they do, you can just run away to another system anyway. Very behind the scenes watching everyone style of gameplay, although the faction mechanics might be a bit complicated.

2. Unfallen/Lumeris. Starting off with Pacifism politics means you can force peace with other factions. I always find this mechanic *HILARIOUS*. Lumeris are also slightly better since their focus on dust makes it easier to maintain Pacifist politics *and* do a whole bunch of other stuff. Unfallen are super slow at expansion, imo, even though they also have a pacifist-aligned populace, they don't really do much other than being pacifist and hard to invade. The point, imo, is to avoid war in the first place, not make it harder for the enemy to win. I don't really like them aside from the funny lore of tree-hippies trying to get everyone to get along.

3. Everyone else. They're all neutral at Pacificism. Honorable mentions for Nakalim, since if you play them right, they have a massive head start, and Horatio, since ecology politics means you can colonize planets that other factions don't have the tech for. Horatio Juice FTW.

4. Cravers/Hissho. It should be obvious, but factions focused on warfare don't do too well at non-combat. Cravers need to conquer/colonize new planets since their planets slowly deplete resource production and they need alien pops to enslave for depleted planets. Hissho need to win battles for their faction resource, completing quests only doesn't quite cut it.

I agree, warfare is by far the weakest part of Endless Space and tbh, 4Xs in general. But if you want to avoid war, umbral choir is your top choice, since they don't have to do anything to avoid war. Otherwise, you'll want to focus on Pacifist politics in the senate so you can neutralize potentially hostile neighbours diplomatically.

tyty

UC, Lumeris and Unfallen are good picks for non-combat, though I would add nakalim whose inflyence bubble allows you to not fight wars but rather take systems through influence conversion, no combat required.

Most factions, apart from Cravers and Hissho can win the game without engaging in combat. Sophons are a good pick, just sit there taking tech after tech and you eventually win a scientific victory (I got my no win in space battles achievement with them). Vodyani can get essence fairly well without going into combat, no leech required. United Empire can just sit in the corner and I would say go for either eco, wonder or science victory. Any faction can get one of those victory types. Riftborn are also a good pick for a wonder victory, that influence is enormous.

Id put Riftborn as a top pacifist contender. Being a Robotic faction, theyre the best "wide" Empire in the game due to being able to colonize Sterile planets(which are Ash, Lava, Desert, Toxic, and Barren) from the start. Sterile planets are rich in Industry, Dust and Science or the three main components Riftborn value as they dont need Food to survive(Other Major/Minor factions will but not Riftborn). If left to their own devices, particularly in manufacturing their population that grant +5 to IDS, they can snowball to an easy victory without having to fire a shot thanks to Singularities. If they do have to prepare for War, theyre one of the best Industrial factions that take advantage of the Energy bias in combat(not to mention Singularities that give them a Combat edge).