TO CLARIFY:
This is my opinion of the matter, so what I say may not match yours.
My top 2 are the Vanguard and Sentinel, with Vanguard just a smidge above Sentinel.
My least favorite are the Infiltrator and Adept.
That leaves the Soldier and Engineer as middling.
Put another way...
1) Vanguard
11/2) Sentinel
2) Soldier
also 2) Engineer
5) Adept
6) Infiltrator
Vanguard fits my playstyle almost perfectly, and so does Sentinel. They each cover all the bases, but in different ways.
Vanguard powers through any enemy, and sets things on fire, while my squad-cryo teammates freeze everything else. (Flare)
Sentinel shrugs off otherwise fatal blows, and has access to THE most damaging combo in the game all by itself. (Warp Ammo)
The reason Vanguard wins out (by a hair) is because Biotic Charge is amazing. Tech Armor is great too, but Vanguard was my first class, which is why it has the top spot.
I play the Soldier with only a Heavy Pistol. Because I love spamming Concussive Shot with the Amplification upgrade. Ironically, as the Soldier, I use the LEAST amount of ammo, because being able to shoot alternating fireballs, lightningballs, or snowballs is just too much fun to go back to boring old thermal clips.
The Engineer I find to be an ideal "low-level" class, because I only use Overload and Incinerate, so I don't need as many points to maximize them while alternating between the two for back-to-back combos. (Armor-Piercing Ammo)
As the Adept, I find the Singularity to be too small, and it never hits where I want it to. I find Liara to have a better Singularity.
Since squad powers activate instantly, it goes where I want it, when I want it. (Warp Ammo)
And finally Infiltrator. The combat is too fast for effective sniping, and trying to capitalize on the damage boost from cloak adds too many steps for it to be effective for me. By the time my attack is "set up", the circumstances have changed and my attack is now useless. (Defense Matrix)
All together, I find the combat most suitable for a Vanguard (weapons and biotics) and a Sentinel (tech and biotics). Adepts have "too much" biotics for me to use effectively, and Infiltrators don't have the "correct" combination of weapons and tech. I don't bother with the Engineer's drones, and the turret is either too fragile to be of use or by the time it's set up I've already dealt with the threat.
Don't get me wrong, firing Incinerate from cloak while remaining invisible is nifty, and lifting an entire squad of Assault Troopers with Singularity and detonating them all with Throw is cool, but I feel they are too specialized. If I'm surrounded, cloak only lasts for so long, and if they're shielded, my biotics do virtually nil.
If you're out of your element, you're up the Omega 4 Relay without an IFF.
Vanguard and Sentinel are adaptable classes. Sentinel by design, and Vanguard by synergy. Regardless of enemy type, I can control the battlefield and put the hurt on the bad guys, whether by Charge/Nova or Overload/Warp/Throw.
All things considered, there's my two cents.
I should go.