Well, I would say either Stormtroopers or a Hardened Veteran Squad.
Hardened Veterans have an excellent variety of weaponry to choose from as well as being able to take a Veteran Officer w/ 3 wounds. Not only that, but you can take a chimeara to boot! This allows you to approach different tactical situations and have reasonable success. Unfortunately, these guys only have 5+ saves and the squad can get expensive if you're not careful.
Stormtroopers if chosen, should be used to deepstrike as they can take out annoying tanks and tough footsloggers (terminators) with their meltaguns. Once again, these guys only have 4+ saves and are pretty much limited to what role you want them to play (little versatility).
So it depends what you need most in the army. If you need just more firepower and a flexible unit for any situation, choose veterans. If you need a specialized unit, go with stormtroopers.