Gotta go with the Dark Angels. I always painted my armies in the pre-Heresy scheme from the Rogue Trader book (black with white detail, and I often used red as the tertiary color). Any of the listed chapters are pretty cool, though. I just wish that someone would write Space Wolves fluff that didn't make them sound like morons most of the time, or at least very 1-dimensional characters. Even Dan Abnett couldn't do it in Prospero Burns, which is saying a lot, since he's one of my favorite BL authors.
If I have to choose I'd say Dark Angels. I hate Ultra Smurfs, have no need for puppies in space, and if I ever thought about playing Blood Angels, well Chaos does it better. But if I had my way, Imperial Fists all the way.
I started a Dark Angel in 3rd Edition, loved their Stubborn Rule to no end, thou shall not flee but blast my boltgun at the enemy! Though I like the Ultramarine colours so I'm stuck between the two. Most likely do both.
Dark Angels, due to the fact that they are one of the first, Still-Loyal, Horus Heresy chapters, and they also looks pretty damn bad-ass with their cloaks.
