Honestly, I love every marine unit in Dawn of War 2. The tactical marines offer a tremendous ability to soak up fire (having more HP than three squads of guardians if a mere fraction of the firepower), and early in the game their flamer upgrade ensures simply massing units is a foolish gambit. Scouts offer the best defense the marines have against a melee charge early in the game with their ludicrously lethal shotguns. Hell, if an ork or tyranid player isn't controlling their various melee squads proficiently on the individual level, any squad in the tiny starting army will bring utter ruin in short order. The Assault marine is the solution to most tactical dilemmas, especially when an army is foolish enough to rely on weight in firepower to win the day. The Librarian is easily as effective as any commander unit on the field and is especially powerful when confronting armies that send large numbers of flimsy units into battle. When a dreadnought enters the field, victory is generally all but assured as the thing will absorb withering fire and deals massive damage, and when properly supported only a well played tank or super unit is really sufficient to bring them down. The predator, while far more lethal, is also far harder to keep alive and thus I only field them when melta grenades, plasma fire and missiles are insufficient to turn back an armored onslaught. And, should terminators take the field, the battle is almost certainly over. I can, in fact, only recall a handful of instances where I have lost a battle once I manage to deploy these things. Nearly impossible to kill and armed with weapons suited to murder just about anything, these guys are certainly the stars of the show.
Even the commander units are awesome. The force commander is a melee powerhouse who can absorb tremendous punishment while delivering it in kind. The Techmarine might be flimsy but there is no deadlier ranged unit in the game. Hell, alone a properly played techmarine can bring down a fully upgraded squad of marines in seconds alone - a feat other races would have to use an army to accomplish.
The eagle eyed will note that I didn't mention several units. I do like the devestator squad, but I generally prefer to use a turret for the job (and I play as a techmarine as often as not). The razorback offers incredible anti-infantry firepower in a highly mobile package and gives you troops quick transport options but it's place in the game generally relegates it to special purposes. If I had plenty of resources, the dreadnought is the unit of choice and if resources are scarce, I would rather use them to upgrade existing units. It is only when I reach T2 long before the other side (and thus armor maintains a relative degree of invulnerability) that I use it, or when playing on the larger 3v3 maps where quick transport is often advantageous. The plasma cannon, while hilarously deadly, is certainly the least useful of the artillery weapons and often the job is better done by simple assault anyway, but sometimes a prepared defense is impossible to maneuver around and dug in well enough that an assault would be disasterous.