Here is my list, from hardest to easiest with a really short explanation.

1. Assualt.
Hardest to play as simply because you are the only one who usually goes for the objective. The assualts damage is very low, in fact unless you got a full 50 assault almost anything has 1v1 advantage to you. Compliment this with the fact that only giving the class 1-2k extra health as a tank? WHAT THE HELL. 1-2k extra health is like nothing. Even the defense buffs are really pointless since any class has debuffs, even the medic making your lil extra def pointless. Destro assaults are fun, but kinda weak. Even if you put all your attacks at one turret you can just barely kill it.

2. Medic.
Healing is nice and all, but really you can't rely on your teamates for merc at all. Poison medic>regular medic. Simply because healing is ok, but you do pretty much equal healing as poison. Its better to have a damage option case your target dies on you because he was too stupid.

3. Recon.
Recon are pretty easy. Snipercon is best, since you can hit 1-2k per hit. It reduces healing, and its pretty much impossible to not get hit. Although meleecon and bombercon are pretty lacking, i find them to still be useful. The main problem i find with recon is that medics don't heal you. So when you take damage, you are sincerely fucked.

4. Robotics:
EASY TO THE MAX
You can:
-Heal yourself, continually.
-Build a turret with more damage output than ANY class.
-And you can heal all your little buddies at once.
-Best part, you can put turrets that take forever to kill, up every 30-45 seconds!
Robotics are easy, and people SUCK at them in low level mercs. On offense they put them defensively, on defense people use personal turrets from far away. Still, self-sustaining, and easy stuff. Not really a downside to this class unless you count your "sorta" flimsy without your tools.