Harlan Teklad is considered the best most nutritious and balanced lab block out there. It cannot be purchased at stores but must be shipped in. You can order it from Kim's Ark Rescue, or if you are nearer another supplier Kim's Ark will redirect you.
The next best one is considered the Mazuri. I use Mazuri 6F myself (16% protein, but in 50 lb bags), but PetSmart only sells the 2 lb bags of 5663 (23% protein).
Hagen Nutri-block are considered okay.
Oxbow Regal Rat is expensive and some rats just don't like it. Its not a lab block but has kibble in it.
Stay away from Sunseed Critter Cubes, Reggie Rat and anything Kaytee.