So I guess it can be assumed that their are a fair few people here on this forum that read, play, or just know about war hammer Fantasy Battle/age of sigmar/40k/etc so i guess that calls for a "roll call" of sorts with a simple questions, whats your favorite faction and why? For me it's the Skaven in fantasy (which i also use in 40k as a IA:13 heretics and renegades army :P) and the Imperial Guard in 40k, and the same reason for both, hoards of models, and more dice being thrown than I could ever afford.
Imperial Guard. Everyone is always going on about their Space Marines but it's easy being badass when you're a genetically engineered super soldier with bolters and power armor. The IG is just a bunch of regular guys with shitty gear, going up against everything from super-advanced aliens over eldritch abominations to British football hooligans. And when they have proper support in place they can kick some serious ass too.
Second place goes to the Orks for being, well, Orks. You have all these factions fighting for the emperor, their race or to get power from the Dark Gods or whatever and then you have these guys who fight simply because they like fighting. I also find it amusing that they are literally a race of football hooligan fungus.
40k = Night Lords. I love this insane lunatics <3
Fantasy: Orcs and Goblins and Vampire Counts
But in my opinion wents GW more and more nuts so I skipped 40K and Warhammer Fantasy is no more (Age of Sigmar is just dumb...). So my money now goes to Warmachine Hordes (Skorne).
Mine are Space Wolves from 40K
Reasons
1). They the only loyalist chapter to have a "disagreement" with the Inquisition , including the Grey Knights and survive. Kindly sending them back where they came from.
2). Another "disagreement" with the Ecclesiarchy.
3). They think the codex is for fools.
4). They got hair and ride on wolves bigger than your car.
5). They drink then start fights.
and I could carry on forever.
Custom factions, especially the likes of the Angry Marines. No contest.
Are there Skaven in 40K? If not, that is the biggest heresy of them all.