I like Codex Space Marines. It is a good generic codex. It is filled with good general purpose units, that are pretty darn good at everything, without much in the way of real weakness. The worst things people tend to say about Space Marines is that while Tactical Squads are okay, they aren't Terminators (or Sternguard, or name your favorite Ultramarines players gripe). This is from people who play Space Marines, of course. Anyone who plays another Codex, especially a xenos one, will tell you that GW loves them some Space Marines, and that everything Space Marine is made of win, and that all gene-seed is extracted right from Chuck Norris's awesome gland.
I, on the other hand, love Codex Space Marines because of Scouts. As always, I guess this goes back to my failing of only loving theme or gimmick lists. Heaven forbid I have a flexible, consistent list. Give me Deathwing, or Biker Orks, or LoganWing, what have you. Scouts have more inherent flexibility for how they are played than any other unit I can think of. They can be close combat, with knife & bolter pistol. They can be sneaky, with camouflage & sniper rifles. They can be shooty, with bolters. They can be long range, with decent special and heavy weapons options (I do love some special ammo for a heavy bolter). They can even be air assault, with Speeder Storms. That last is what got me looking at them again, what with Fritz gimmick list that I detailed below. Of course, the weakness of said list is that it is, well, a gimmick. It would work well against a journeyman player who was not looking for the tricks you were about to throw his way.
Against a good player, or someone who has faced the list before, and knows what to expect from the early game, I envision getting chopped to pieces and having my terminators walking around trying to get to grips with shootier enemies.
In any case, another great thing about Space Marine Scouts is that with all the special ways to outfit them, you can use them with 'counts as' just about anything. I was thinking of having scouts count as ODSTs and using Spartans for Sternguard, or some such. Of course, there are a million and one things scouts can count as, what are some of the things you have thought might be good for them? Comment below!