Let's organize the skills, and not by tree:
Offense: Impact, Metal Storm, Overload, Scattershot, Assault, Sentry, Guided Missile, Grenadier, Deploy, Refire, and Barrage.
Defense: Defense, Quick Charge, Fitness, Grit, Stat, Cauterize, Stockpile, Aid Station, Supply Drop, and Revive.
Turret offense: Sentry, Guided Missile, Deploy, Refire, and Barrage.
Turret defense: Stockpile, Aid Station, Supply Drop, and Revive.
Altruistic: Cauterize and Revive. These skills provide no benefit to yourself directly. You cannot Cauterize yourself with an explosive and the turret despawns when you're downed.
Any other skill, I'll call either personal offense or personal defense. The Scorpio Turret action skill is a given. Scattershot or Assault depends on whether you prefer shotguns or rifles, respectively.
I ordered them in a specific way and split turret skills into their own sub-sets because, grenades and the turret are an afterthought.
Grenadier: Grenades are iffy. Grenades are iffy for several reasons: (1) They can backfire on you, particularly the bouncing betties, and this can prevent you from advancing further until it's done exploding as well as hindering your vision, (2) there's a bug where terrain will cause the grenade to disappear, (3) if it's caught in a bad spot, its blast radius may be ineffective, (4) their damage is delayed, which is bad in comparison to how you probably could've killed the enemy sooner by focusing on gunning, and (5) considering how the Grenadier skill and the Warmonger class mod specifically up grenade damage, this is the only thing that Roland has that boosts and resonates with grenade damage except for shock with the Shock Trooper COM.
On the other hand, Roland has the only directly grenade-oriented skill, and not to denounce grenades completely, grenades still have their place...in mobs. Grenadier depends on how often that you use grenades. And having at least one point in it means there's reprieve for the obsessive ("must have full ammo"), while being more hands-free and offensive than Supply Drop.
Turret skills: The turret is similar to grenades. It's delayed, it's location is confined, its damage isn't easy to boost, and if you've developed an attachment to its resupplying ability, limits your movement. Furthermore, it interrupts your ability to aim and shoot your weapon to deploy it most of the time, it only fires in spurts, and its cooldown is long, being the only one of all classes that cannot be easily redeployed in seconds, not to mention at the lower levels.
On the other hand, it's an extra gun and it's useful when there are many enemies in one spot. Even if all personal offensive and personal defensive skills are filled out (which comes to a total of 11-1 for preference of shotgun or rifle), that leaves 19 skill points that you'll have to spend on something non-personal.
Cauterize: You only need one point in this to heal your team-mates by shooting them. If you play with others, put at least one point in this.
What are the offensive COMs? Heavy Gunner, Rifleman, Commando, Shock Trooper, and the more exclusive Patriot Vladof, Gunman S and S, Champion Atlas, and Marine. I have little experience with the loyalty enhancement mods and Marine. From my testing, Commando and Rifleman outdo Heavy Gunner and Shock Trooper. I followed Gearbox Software Forum's DaCheat's Shock Trooper guide, but shock Bulldogs just don't proc enough. I've also tried a shock Destroyer and Orion.
Patriot Vladof loyalty enhancement mod: Let's see what I have for that: Vladof Hammer, Vladof Revolution, and The Chopper. Vladof Hammer is better used by Brick and its ammunition feeding device size is annoyingly small, Vladof Revolution is outdone by Atlas rifles, and The Chopper is for PvP. +61% Weapon Damage, Cauterize, Defense, Grenadier, and +56% Weapon Reload Speed. The Rifleman COM adds more damage.
Gunman S and S loyalty enhancement mod: Serpens = rifle. Draco doesn't have enhanced proccing like normal legendary weapons. +61% Weapon Damage, Metal Storm, Overload, Assault, +56% Weapon Reload Speed. Each bonus is a personal offense bonus, which is lovely. The only other major weapon is the Crux, but again, that's Brick's bag.
Champion Atlas loyalty enhancement mod: Havoc, Ogre (elemental challenge), and Ajax's Spear. +61% Weapon Damage, Fitness, Grit, Impact, +84% Weapon Accuracy. C'mere, sweetheart!
Marine: Launchers (rockets). "I wanna be like you, Brick! (Except worse.)"
In PvP, Shock Trooper is viable, because shock is the king element in PvP. This is because a player's total hit-points are mostly composed of shield.
Support: Let me put it like I put so long ago: Tactician? No one's gonna say, "Wow, +47% Shield Recharge Rate and +19% Team Shield Capacity," cock their head with an affectionate smile, "thanks, [insert your username here]!" You know what's noticeable? Ammo regeneration via the Support Gunner COM. You're the only one who can. You love it.
But other than for the open admiration, Tactician's cool. Leader for XP-boosting. You're playing support when you're playing in a lower level person's game, where you're too powerful. You're not if you're equally levelled. Not to forget that errbody loves Support Gunner for general play.
Caveat: If you don't have the good enough gear, one spec may stand above the rest in what is available to you.
That's when the Heavy Gunner COM shines: it applies to all.