I voted for the Mandarin (meaning Ben Kingsley as Trevor Slattery) because out of all the villains, he was the most memorable, most compelling, and most mysterious. Even though some longtime fans disliked the twist with his character (which is understandable), I thought it was a clever interpretation of the Mandarin in a medium outside comics. I also thought that Kingsley brought great elements of the classic Mandarin from the comics while adding new elements to make the Mandarin more unique and more relevant to the MCU. Kingsley's acting really made a difference, giving both the Mandarin and Trevor a wide variety of emotions and mannerisms.
While all of the Iron Man villains had some good qualities, I don't think any of them gave as memorable performances as Kingsley's Mandarin did, and Kingsley's Mandarin seems to have gotten the best reviews and recognition from general audiences and critics alike. That makes sense to me, because the Mandarin was simultaneously eerie and surprising, and this combination made him more compelling than your average movie antagonist.
In addition, I love the Mandarin's look. It's one of the most creative costume designs for a comic book film villain ever. I like the combination of ancient warrior symbols with modern terrorist motifs, which is simultaneously regal and eerie. For all these reasons, the Mandarin is my favorite.