The thing with Leng is that I don't hate him for his character being an asshole, because... that's his character and that aspect of him is well done. My problem is that he sort of just appears in ME3 and starts fucking with everyone's shit.
I also love love the letter Leng sends you after Thessia. I love how petty it is and the obvious inferiority complex he's got going with Shepard. Owning him at the Base was delicious.