It tends to be situational, largely based on group make up and behavior. But there are a few universal truisms, I suppose.
The first is that Reapers are generally the worst. They're one of the best if you're a sniper, or just a bad shot, as their huge, chunky bodies make for easy targets. But aside from that, there isn't much to recommend them. They have the beefiest low level units, flanking Brutes are an ugly surprise, and Banshees are horrifying, fast moving damage soaks that can absolutely decimate an entrenched position. This makes any objective where you need to stand your ground particularly obnoxious versus Reapers, especially on higher difficulties, unless people are feeling liberal with rockets.
The second is that Geth are only bad if you're lazy about checking your flanks. If you get tunnel vision with Geth, you're going to be eating a Hunter/Pyro parade. That pretty much goes without saying. If you aren't lazy about checking your flanks, Geth are the easiest, and by a sizeable margin. They're not particularly dangerous at range, and they're collectively extremely slow moving, making it a simple matter of displacing and picking them apart at range. They also have a propensity for clustering up, making them handsome targets for AOE attacks.
The third is that Cerberus has a wide range of fairly hopeless units supported by a couple of extremely ugly ones. Normal Cerberus troopers are pushovers, and Centurions aren't much better. Guardians are feeble, and the Atlas is bulky, slow and weak. The only things saving them from ruin are Engineers and Phantoms, both of which turn from irritants to group killers on higher difficulty levels. There's nothing quite like rounding a corner into 2-3 Phantoms or a turret party to ruin the round.
So generally, I'm tempted to say Reaper-Cerberus-Geth, unless you're really lazy about checking your flanks, in which case it's Reaper-Geth-Cerberus.