Well, first off, are you collecting
SM army or you are fighting against one?
If you are Marine player, the best marine unit is no doubt the Grav centurion, especially supported by powerful psykers like Tigirius in Librarian Conclave (I tried the librarian conclave multiple times, it is really great force multipliers, and being versatile since you can decide to roll on which psychic table depend on what your enemy have). The second best units is White Scar Bikers, highly mobile and deadly shooters.
Formation wise, the best one is the full battle company in Gladius Strike Force, spam Assault Cannon Razorback, that high
RoF S6 range weaponry is what Marine armies lacked, and being rending is even more powerful. Second best is the Skyhammer formation, deadly alpha strikers but it is more or less like suicide units, so you need other units to maximize the advantage of their havocs wreak upon the enemy.
If you are playing against marines, remember to get either high
RoF high strength fire power or lots of attacks in combat or
AP 2/3 weapons, every lose of a marine model is a pain due to their relatively low model count army nature. Meanwhile, hide your high value units in cover, Marines major flaw is lack of proper weapons to flush out enemy units out of cover short of a few sets of Flamer / Heavy Flamer. Their Lascannons / plasma weapons / meltas / Grav weapons are powerless when Carnifexes have a toe in ruin while a venomthrope is just 6 inches away.