Honestly this won't be as long as Spcae Marines don't have many choices in either squad...and even less good ones. Still you need troops and there are some decent options so here we go.
Troops
Here we have Tactical Marines and Scouts. These are both very symbolic units in 40k in general and there are few people in the hobby that don't have a Tactical Marine lying around somewhere in their collection. Both of these recieved major changes for better and worse with the coming of the not-so-new codex, but since you need at least two from these guys, you have to know what to do with them.