3. I'm scared to say this but............................ Krogans. I can dominate with the Warlord but he's super easy to use... right?
He's a slow character with powers that are entirely melee-focused, with a complete inability to dodge or take cover or even vault over it. Unlike the Juggernaut, he can still be sync-killed; nor does he have any ranged powers. He's rather hardy and can deal out massive damage once he's in range, but depending on the map and team enemies could have been killed by teammates before he even gets there.
Honestly, among the Krogan I'd rate the Vanguard as much easier to use (in the absence of terrible lag, anyway). He lacks the health regen and doesn't have the same base health/barriers, but a charge will restore his barriers (completely if you take 6B, which scales very nicely if you're using a Cyclonic) as well as staggering or killing up to three and moving you into easy shotgun range.