Me1 - I considered the Vanguard and Infiltrator to be the best.
Infiltrator = Engineer - (Hacking & Medicine) + (Sniper Rifles, Medium Armor, IMMUNITY)
Take hacking as your bonus skill, and you have an engineer with sniper and way better durability. I considered ME1 engineer to be the overshadowed because of this, especially because Immunity is not available as a bonus skill.
Vanguard = Adept - (Stasis & Singularity) + (Shotgun, Medium Armor, and Adrenaline Rush)
Take stasis or singularity and you have an adept with greater spammability due to AR. You did miss out on bastion/stasis, but I don't think this is a compelling reason to pick adept over vanguard.
Soldiers in ME1 were effective but not too exciting. Having to spend points in each weapon you wanted to use meant you probably only used 1 or 2 weapons. With unlimited ammo, having every weapon wasn't as big of a deal.
ME2 - classes are actually a lot different from one another with less skill overlap. This is also helped by the fact that the loyalty/bonus skills aren't just skills from the other classes. Some classes may be easier to play or more powerful, but none are simply overshadowed as they are in the first game.
Having multiple playthroughs, I would consider the soldier second only to the sentinel in terms of general effectiveness. Adrenaline rush shouldn't be compared heavy charge or the sniper zoom slowdown. Both are much shorter duration and have very specific limitations. AR is good offensively with any weapon due to the massive damage bonus, and it can also be used defensively to retreat.
I can understand the dislike ammo powers since these do feel like they should be weapon mods. Weapon specific skills (like in ME1) would have had a better flavor. However, ammo powers are essentially passive boosts and therefore don't chew up a cooldown that you'll want to use for AR or your bonus skill. They also help make soldiers a well-rounded class by giving them the ability to incapacitate as well as tools against shields, armor, and health.
Now, concussive shot in ME2 was pretty bad. It was like a combination of a poor man's throw and a poor man's warp that was only decent against barriers. Worst of all, it got no bonuses from any of the upgrades. A useful concussive shot, which we'll hopefully see in ME3, would give a reason to hit something other than AR every cooldown.
Having ammo powers as passives and only AR for the cooldown, a non-ammo bonus power could potentially see a lot of use.