So I would like to know if you had to choose which Chapter (Legion as well for Chaos lovers) is your favorite, which would you choose and why.
I'll start: I enjoy the Black Templars mainly due to their play style with a bit of their fluff to get me initially interested.
When in doubt, throw more men at it! A Guardmen taught me that once and it hasn't failed yet.
The Black Templars in general suit my favored play style of moving gradually up in a well planned and overwhelming offensive. They might not be as fast as the Blood Angels but they bring a lot of bulk to the table since their squads of infantry range from small with big guns to large with recruits (read meat shields) to bolster their ranks. Add to that one or two cost effective and dangerous melee combat commanders and you have THE offensive infantry force. Now if only it didn't cost an arm and a leg to get that many models I would be able to play them outside of proxy games.
As far as the Fluff goes they are pretty neat as they tend to be actions over words mixed with stubborn commitment and dedication. Also never let it be forgotten that they are teutonic knights in space that are determined to get shit done in their lifetime with constant crusading (yet they will never be Ultramarines the greatest of them all...). Their named characters are also pretty neat as they generally involve self improvement in the face of great adversity rather than being the best by default.
Thousand Sons, their lore and backstory are first rate and the rubric marines are awesome entirely unto themselves. Plus they seem to be the only traitor forces which seem to realize that getting the hell out of dodge before the exterminatus fleet rears its head up and drops a few cyclone torpedoes on their heads is a fundamentally sound strategy. Plus that one minor incident when Ahriman the Exile (guess where I got the username from) turned a planet into a giant daemon summoning ground and turned countless marines and guardsmen to dinner. Took the fucking Grey Knights to clear out the whole mess (http://static.comicvine.com/uploads/original/11/113883/3260095-7486225179-32599.png).
Honorary mentions to the Iron Warriors, why? Because royally pissed off combat engineers with enough artillery to flatten a hive world. And Sebastus IV, that was hilarious.