Damn, late to the party. Still, my two cents.
It all comes down to play style.
I've played a fair bit through all three games. Classes I've gone the distance with are the Adept (one playthrough), Infiltrator (2 playthroughs), and Vanguard (2 playthroughs). A playthrough is games Mass Effect 1 to Mass Effect 3.
Adept
Pros
- Space magic
- Spell Combos and Explosions
- Safe behind cover
- Straight forward and easy to learn
Cons
- Stationary
- Very little variation
- Relatively weak armor and health
Personal note. One of the more boring classes. Not my play style. Sit behind cover, throw out your magic, rinse and repeat. Played on Veteran.
Infiltrator
Pros
- Cloaking / Get out of jail free card
- One shot, one kill
- Time Dialation for easier headshots
- Straight forward and easy to learn
Cons
- Relatively weak armor and health
- Stationary
- Possible ammo problems on higher difficulties
Pesonal note. Enjoyed the class. Cloaking plus time slow plus headshots never got old for me. You can mow down entire ememy squads before your team mates can react in Normal mode. Even Insanity bosses were on the easy side with the heaviest weapon and all the upgrades. Played on Normal and Insanity.
Vanguard
Pros
- Fast, Mobile gameplay
- Strong Weapons
- Regenerate Shields on Charge
Cons
- Advanced class, trickly to learn
- Easy to die
- ME3 instalkills
- Lack of long range damage
Personal note. Hands down, my favorite class.
Mass Effect is the first and one of only two cover-based franchises I have played, the other being Spec Ops: The Line. I find the gameplay generally boring and stagnant. It is the reason I found Adept a chore to play. Infiltrator with cloaking let me move around a little, so it was more bearable. However, sitting behind a chest high wall and playing duck hunt is not my idea of fun.
The Vanguard was a much needed breath of fresh air. The gameplay is smart yet frenetic. Split second decisions and risk taking is the name of the game. Biotic Charge is a godsend ability. The class is so much fun for me. In Mass Effect 2, get the Claymore, Max out charge, get whatever cooldown specs you can, and go play. In Mass Effect 3, keep your weight as low as possibly for Mmaximum cooldown. Charge and Nova. Played on Veteran and Insanity.
Of note, you can glitch out the game in the form of less enemies. Examples are multiple elevators in Mass Effect 2, and 'Storming the Beach' in Mass Effect 3. If you can Charge along fast enough, you'll hit triggers that move you forward in the scenario, effectively halting planned additional waves of enemies. Turned into sort of a personal challenge on how fast I could go. Reward was less enemies!
As mentioned, you'll find hiccups in Mass Effect 3. Especially against Banshees. Charge -> Melee Instagib can get incredibly frustrating. But the fun far outweighed the frustration.
Anyways, enjoy what you play. My vote would be the Vanguard.