I run a 7200mah SMC that has got be up there as far as max capacity you can fit. I can get 45 or so minutes out of it depending on how much of an ass I am being. Looks like the version I have is no longer offered in lue of one with a higher C rating so verify it will still fit before going that route.
You can can get super high capacity batteries but they cost a lot more for not much more capacity. I would personally rather have 2 5000mah packs than 1 7000mah pack and the 2 5000 packs will probably cost about the same as the single higher capacity pack. The price rises pretty dramatically after you cross the 5000mah threshold.
Looking at the SMC site, the price disparity isn't as bad with the 2s packs as it is with the 3s packs that I buy. If you're looking at a 2s and are only getting one, that 7200mah 90c shown above is a great deal.
This probably isn't the best, and it won't give you the longest run time, but the GForce batteries at Value Hobbies are a good option. I picked up two 4000mAh 25C for under $18 apiece. Fits in the granite, but unfortunately does not have a Deans connector. Not a big deal to solder on a new one if you know how and have the equipment, so not sure if that would interest you.
Edit - I tried to post a link to it, but it wouldn't let me. Probably cuz I'm a new member.