As far as the poll goes, I love the absolutely insane level of customization in Saints Row, whether it be cars or clothes.
What I love most about Saints Row, though, is the humour. The first game had almost everything the sequel did, but didn't know what it wanted to be as it toyed between being a unique experience that didn't take itself seriously, and a GTA clone. Saints Row 2 hit the nail on the head in terms of direction by letting its sense of humour shine through, and was much more enjoyable for it.