I need a Crank Reccomendation. Single Speed, BB30
As mentioned in some comments, many cranks will fit. You need a 30mm width spindle crank. BB30, a cannondale staple crank size, is just this. 

You can use a 24mm crank with an adapter. Some cannondale riders I know have all their bikes set up like this. I don't really like it. Extraneous parts and more weight. The advantages are serviceability and possibly less creaking, though I've never had a problem with this. It's more serviceable because you have a threaded bottom bracket that's easier to remove. (Most home mechanics don't have the proper bearing pusher/pullers to remove and reseat BB30 bearings correctly, though I've seen it done).

Sram is what I use in my cannondale and I'd recommend it. I'm sure BB30 cranks from other manufactures are just as good. The bike came with an FSA crank, but I didn't care for it because it is comparatively heavier and less stiff than my Sram Red BB30 crank. 

Here are some pics of what it looks like in the frame: