In the most practical story sense they make for flat, dull, boring, and completely uninteresting characters. In most cases I can't even credit the writers for making a character that is fun to hate, because they usually stop being characters at some point and just become obstructing and frustrating plot devices.
DA is pretty balanced for the most part. I loved the character of Meredith up to the reveal that she was possessed by her sword.
You got me thinking about a politician in ME who's fun to hate. Not gonna day Udina cuz that's too obvious. The only time he was really any good was during the renegadeshep speech at the end of ME1 if you let the council die. There's his true nature. And I gotta say, I got a real sense of how ... final... everything in ME3 was gonna be when Udina did a 180 degree personality change in the first part fo the game. I felt bad for the guy that they didn't let him be the one to tell shep his spectre status had been restored. They shouldn't have had Sparatus turn back as he was leaving and say, "oh by the way..."
Humanity will always be considered second rate, indeed. Should have waited until Sparatus left, and then have Udina turn to shep and quietly, but intensely, say, "Shepard, you're still/you are again a spectre. You've got free reign to go out there and do what needs to be done."
Uh, anyway my pick was gonna be Sparatus. In ME1 he was a typical turian anti-human ****head. I was content to hang up on him. ME2 made him famous, air-quoting away the reapers. He was the perfect one to hate. In ME3, though, he developed as a character too. In the first meeting with the councilors he was the only one willing to consider the crucible idea. He came to shep with a way to bypass the council. After the coup attempt, he's the first one to say, "You have saved my life twice now. I owe you a great debt...." if you don't kill off the council in ME1, that is.
Finally I decided on Valern. Valern has no redeeming qualities in any of the games. Trouble is, I don't love to hate him. I just hate him. Heh.
And I could never hate Tevos. 