Least Favourite Loyalist Legion: Ultramarines. I just don't find anything the Legion/Chapter has done to be that great or well written. I just get this vibe from them "we sat this one out pretty much so we outnumber you all but we will be nice and create parity even though ultimately all our fancy colours will join us in our new centre of mankind should the need arise" then "oh snap nids, good luck everyone else were being attacked and it isn't pretty!"
Least Favourite Loyalist Primarch: Leman Russ. I just don't relate too him as someone I would want to follow into battle, or even would think he could understand what he was doing. He seems like the brother who is a bit slow and just a loose cannon... "Hey Russ he just called your momma fat" as he goes full out war without even taking a second to think.
Least Favourite Traitor Legion: Emperor's Children. I didn't like the arrogance in general with the way they are described. They come across a pretty boys too me, and then become a bunch of perverts. They remind me of a bunch of sleezey guys in a bar that just go around trying to get in any chicks pants.
Least Favourite Traitor Primarch: Alpharious. You lead a pretty cool described and operated legion, with real tactics and thought yet you and your twin come across like a lesser clan Skaven warlord, cowardly and forgettable compared to well really every other Primarch.