Favourite?  I've got to pick????  Uh…Salamanders are pretty cool.  The whole 'we are the hit men of the High Lords' shtick the Minotaurs have going is awesome, what with their large supplies of Termie armour.  But generally I prefer the Great Crusade legions who eventually turned traitor - Thousand Sons, Emperor's Children, and obviously the Alpha Legion.  Least?  Ultras.  Fluff and everyone else depicts them as these great noble and completely looked up to heroes, overlooking all their failures and grim-dark potential.  Or at least the new fluff written by our friendly neighbourhood fan-boy Ward - the Ultramarines books are good, and if you want to see well done Ultras read the Dornian Heresy Alternate Universe version of them.  So I guess my least favourite is probably the Imperial Fists - I just don't ever really remember them, or regard them as that interesting.  Dorn always strikes me as the worst sort of idiot, and their strange masochistic tendencies don't really make them sound manly so much as nucking futs.  

Reading Fall of Damnos provided a good insight into good Ultramarines fluff. There's a clash in the Chapter between those who are arrogant yet magnetically charismatic (Cato Sicarius), and more tempered minds like Tigurius. You can see what the short bits of fluff write ups are trying to convey, and it's well done in Fall of Damnos.

 I'm going to have to split mine out. Of the first founding Chapters, all of the Siege of Terra chapters are my favourites, just from a model standpoint the White, Yellow and Red are standout colours great on small models at a distance (not so good for stealth but that's far from the point). And of them it's the Imperial Fists are my favourite, they have a huge amount of history without being obsessed (sure the UM's fired on them, they got over it. An ancient rivalry with a Chaos legion but still hate all traitors equally. A spiritual turning point without it being a 10000 year Emo fest) a speciality without being a one trick pony. Plus when they split for the second founding each of their successors got an almost equal billing, often surpassing other first founding chapters.

Worst chapter though, and at it's core it's a apparently a common problem with the Dark Angels, is the Angels of Absolution. It's a nice colour scheme, but what gets me is that there's this Dark Angel obsession that we all know about (fine whatevs) and the AoA have got over that which leaves them with…. Well nothing really, I've not seen write up that mentions anything other than they kind of aren't as bothered about the fallen as other Unforgiven.

I forgot to mention in the worst chapter ever : The Blood Raven!!!!!!

This chapter should have been wipe out for their lack of loyalty and their heavy tendencies to worship chaos gods….

The games are fine but my god this chapter is lame. Sry for all fans…

And in the favorite side I can add Iron Handsd ( because I'm a former great cyberpunk/Shadowrun player and GM), Carcharodon because I like the hunting way of life in the vast darkness far from imperial core.

 Just a quick note here: FW did not "create" the Carcharadons. The Space Sharks have been around since the (original) Rogue Trader era.

Ultramarine are my favorite. Have been from the beginning and I've remained loyal to them ever since. 

Full-length stories about Space Marines do them no justice; once you get to know the character and see the character in action, everything else is a broken record. Short stories do a better job of relating the uniqueness of any particular Marine.

Least favorite is the Rainbow Marines. Sadly, originally, they "spawned" the Ultramarines.

Angry Marines are clearly the best.  http://1dchan.org/wiki/Angry_Marines

But if I'm stuck with canon, Black Templars all the way, because they get things DONE, righteously and with no mucking about.  Plus they only get angrier when you shoot at them.

Least favorite is the Ultramarines, talk about a vanilla group of super soldiers overflowing with "Mary-Sue-ism" and lacking any sort of interesting character flaws.  Plus their armor is a lame color.