That's a tough one. Surveyors fill me with rage, but they are pretty easy to take out. Stalkers are wicked annoying, but once their shields are down they usually go down quickly, and rats move around like nobody's business, but landing a head shot on them and wiping them out is a great feeling.
I would have to say stalkers because they usually surround you and go invisible often.
SCREAMINGBEATSS posted... Books? Knowledge is priceless yo