If you're willing to splurge, and want a REALLY good cake, I highly suggest Whole Foods' bakery on Roosevelt. They've done a great job for me on custom cakes before. They're esp. good if you love high quality chocolate cakes or cakes for those with special dietary needs. The staff are always helpful, and if you give them a few days or a week's notice, they're able to whip up something extraordinary.
Hope this helps!