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.
To order direct from HT go to their 'contact us' page, but they only sell in the bulk quanity, I think. Rescues sell small amounts, like Kimsark and many others. Some online store do as well, like the rat shop.
2104 - 14% protein good for older rats
2018 - 18% protein good for growing rats and older rats
8604 - a lot of people still use but it is 24% protein, and newer research has shown that lower protein levels are better.