Haha, so many awesome heels in the last 10 years. But, if I had to put my finger on a top 5, it's not that much of a contest.
Triple H: I loved to hate him, whether it was when he was against The Rock at various times, or in Evolution, he was an awesome heel that just had you hating him, and he was good at it.
Chris Jericho: One of those few guys that go good as a heel or a face, and does an amazing job on whatever side he is on. I loved him during the Invasion storyline with The Rock, oozing of charisma, both of em, so it was an amazing rivalry.
Undertaker: During his badass stage, he was quite a good heel, and I did hate him, even though I respect him at the same time.
Rowdy Roddy Piper: Just one of those older classic heels that you loved to hate, he really, honestly defined what a heel truly is, and was the first ever heel, who I loved to hate.
As another fairly modern heel, has to go to JBL when he was in his prime, you just hated the guy and all the ways that he would cheat to win.
Noteables; Flair, Angle, Edge, The Rock