I vote for A-Key Kyo. I've loved seeing them for the past couple years, and as much as I realize there is a time when new things and performers should be introduced, I also kind of feel like A-Key Kyo have become synonymous with Kumoricon (for me). I love the band, and they put on a fun show.
I also think the Slants were great, and they do indeed know how to rock the crowd. I'd vote for both bands, they both fit in the schedule well this year, and I'm sure they could do the same next year. And as far as I know, there aren't any other musical guests banging down the Kcon doors, so to speak. Now if Moi dix Mois were to come through....