With the release of up to date FAQ for the older space marine codex, Space wolves, Blood Angels and soon to be release Grey Knights codex.. Its easy to see people moving to these Space Marine army than sticking to the Vanila Codex Marine.
However in this grim moment, our fellow Vanila Codex Marine still have things that can make them a formiable fighters.
Assault Terminator: Their 2+ sv and 3+ invul safe from storm shield is still a deadly force to reckon. Although other dex have their own form of advantages but the Vanila one remain the simplest to wreck havoc.
Land Speeders: cheap and sturdy, A choice of weapon like Multi melta for suicide Av14 cracker or Typoon launcher for long range support.
Scouts: They are actually a good choice for troops if you want to go cheap on troop choice. A choice of Sniper scouts for infiltrating top floor objective while have a 36" sniper rifle to shoot stuff(remember it rending so can headshot terminator if desire) and camo cloaks which give you 3+ cover(2+ if you go to ground) which give them insane survivalbility.. Also you may opt for shotgun and closecombat weapon combo which you can take advantage of infiltrate, scout move to get into close combat at turn 1. Best yet is a 5 men squad can already get you a heavy bolter or missile launcher. So great troop on a budget.
Thunder Fire Cannon: this guys are great, for 100 pts you get a this kick ass cannon which can do 3 types of round at a 60" range. From 4 small blast Str 6 , ignore cover round or take dangerous terrain rounds. Also you can fortify defences so placing this at a ruin you choose to fortify can give the thing a 3+ cover. Even if you lose the Cannon your Techmarine can join the war with his servo harness. Twin-linked plasma pistol, 2 Str 8 claw and etc.. and can repair transport while inside transport.. Good value
Kick Ass ICs: Heroes like Magnus Calgar ( Deadly warrior with Eternal warrior and morale test is a thing of a pass and ap 2 storm bolter!!) Lysander (Insanely deadly warrior, 2+ sv, 3+ invul, Str 10 ap 1 hammer!!, Eternal warrior 4 wounds, stubborn... nasty) Vulkan Hestan (twinlinked melta weapon, flamer and mastercrafted Thunder hammer nuff said), and many more.(except Sicarius.. he sucks dont take him.. )
Land Raider: Of cos you need something to get your Terminator boys on the move... It the same as the rest but atleast it all the basic gear compare to certain Codexs which somehow ommit out some of its basic wargear. Land Raider Redeemer(Str 6 ap3 flamer anyone?) Land Raider Crusader(To get more Nasty Terminator ) or Land Raider (got 2 Twin linked lascannon.. even if immobilize can still shoot stuff from far) and all can field multi melta.
Troop on Bikes: So far Vanila dex one of the few codex that can do this.. As a Captain on bike can have Space Marine Bike Squad of 5 or more models as troops. So you can do some biker army if you desire.
Sternguard: plenty type of rounds for their bolter and ability to get combi weapon at 5 points.. Place them with drop pods and send them to enemy armor.. cheap and efficient vehicle killer!
So overall this are the unit listed which consider optimal for Vanila dex as other unit lack potential compare to other space marine codex.