Having visited Lockhart, Luling, and Taylor many times, I'd advise you to definitely include Luling. Lot's of folks place City Market in Luling as #2 on their list of favorites, regardless of which Lockhart market they have in first.

Personally, the single best piece of meat I've ever had was a 1/2" slab of brisket from Smitty's - juicy, tender, smoky, just brilliant. So good I almost wanted to cry.

However, on different days and at different times of day, any of the top markets may amaze or disappoint you. Although the best single piece was from Smitty's, I've had other brisket there that was not nearly as good. I've given up on Kreuz's, as the last 3 or so times I've gone, the meat has been dry and a bit tough. They do have the perfect dessert for serious carnivores, scoops of Blue Bell vanilla ice cream, quite cheap too. City Market in Luling is very consistently good, IMHO. Black's and Louie Meuller's are highly variable, from outstanding to so-so.

Best time of day to go is probably early lunch - 11 AM to noon. Briskets tend to get drier as the day goes on.

Edited to add:

Re: Louie Meuller's - make sure you go there early. I've gone twice in early afternoon only to find it closed (sold out of meat). Although any of them might do this, Meullers is pretty consistent in this behavior.