Whatever is available and depends on how much you want to apply and the frequency.
I like a good cheap 39-0-0 myself. I half rate it, and apply every 2 weeks during the grow in and then back off during be summer to one application.
Milorganite is a favorite, and works well, it's slower to work, it will feed the yard just perfectly fine. Cost is higher too.
Some like a slow release fertilizer, but I'd look at how you maintain the turf, how you cut it, and other factors to see if it's worth the cost.
A balanced fertilizer like 13-13-13 or 10-10-10 would work early, but be careful getting anything the additional p and K in it if you haven't done or care to do a soil test.