If you like to use the jetpacks, assault squads should be your favorite units. They are quite efficient at disposing off large groups of enemies and vehicles - after providing them with Melta Bombs they can stand up to vehicles. My personal opinion is that tactical marine squads equipped with rocket launcher do a better job as anti-vehicle units.
Devastator Heavy Bolter Squad
After securing a strategic point you can easily defend it by deploying this heavy bolter squad. Once placed in the enemy's path they're able to pin down enemy units allowing your troops to move freely about the map.
Devastator Plasma Cannon Squad
If you're high on resources pick plasma cannon squads over the heavy bolters. They're as good at holding enemies back and they deal more damage. They're also able to knockdown with one successful shot and are effective against vehicles.
Without any upgrades this squad is pretty much cannon fodder, but when equipped with sniper rifles (one of the upgrades available) they become a decent support able to pick off enemies without getting into the heat of battle. Their additional perk is ability to repair vehicles and turrets - keep one of those units in your base in case you might want to repair your Dreadnaughts.
Tactical Marine Squad
Squad that's definitely worth your while - they don't stand out against enemy infantry right after training, but equipping it with rocket launchers and/or flamethrowers allows you to fit them to match your opponent's game style. Tactical Marine Squad is a great proof that Space Marines don't rely on the diversity of their units but their ability to adapt.
Incredible infantry squad. If you have enough resources summon them without hesitation. A lone Terminator Squad is able to face enemy infantry without any upgrades. Equipping them with assault cannon makes them almost perfect against infantry, while rockets make them suitable versus vehicles. One of the most important aspects of the Terminator Squad is however, the fact that they cannot be pinned down. The drawback of the Terminators is that they cannot withdraw.
Terminator Assault Squad
As mentioned above, the Terminators are near-perfect soldiers, so it's a good idea to have as many as possible. The Assault Terminators don't use firearms preferring energy hammers instead - this forces them to approach their targets. This can be still worthwhile because once they get close enough they deal enormous amounts of damage making them effective against infantry, vehicles and building. They also cannot be pinned down, but cannot withdraw.
Dreadnoughts make a great weapon against enemy infantry thanks to the assault cannon. Without the cannon Dreadnought fares decently against vehicles. Dreadnoughts work best when supported by tactical squads with rocket launchers.
Very powerful unit that does great in the heat of battle (better than Dreadnought) but it doesn't do well without some kind of support - two tactical squads seem to be the best possible support. Predator doesn't have any upgrades so once it is built, you don't need to spend more resources on it.
Razorback is rarely used even despite the fact that it can be effective against enemy infantry and reinforce allied units. Rather then building Razorback it is better to devote the resources to build Predators and Dreadnoughts.
Despite the fact that it doesn't sport the assault cannon Venerable Dreadnought is way better than the regular one. It does better against enemy units and has higher health. The devastating charges it is able to perform are a huge asset of Venerable Dreadnought. Its main drawback is the price - if you manage to accumulate so many resources you'll probably do better building more of the cheaper units.