AOKP. Hands down for me. Smooth, stable. But what really puts out over the top is the level of customization available.
For the record, I did run CM9 for a while, since it's what I'd mostly heard about before I rooted my phone. But I'd already gotten a taste for AOKP and switched back pretty quickly. I've tried a few other rooms a well, but I'm sticking with AOKP. \0/
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