2. Walder Frey
For this one, I'll try not to give too much away. We were briefly introduced to the very old and vile Lord of the Crossing at the end of Season 1. Catelyn Stark made a deal that her son Robb was to wed one of Lord Frey's daughters in exchange for letting the Northern army cross through the Twins. Robb had to consent, but soon after he naturally broke his promise and married a hot nurse from Volantis, going against his mother's wishes. Despite his frail appearance, you should never break a deal with Walder Frey. I'm betting by the end of Season 3, you'll know why.