I'm of the opinion with Z that the more commonly ordered the model, the higher the quality will be. Top of the list will be non-Forge World Knights and Space Marine Rhinos. From what I've seen on this subreddit, he's been casting Rhinos and Knights in a material that is plastic-like and of very good quality. I don't know what other casts he does with those materials, but I think those are semi-confirmed to be going around.

Personally, I found that my Codex Imperial Tanks came in a lot higher quality than some Forge World Infantry I bought.

Honestly, there's not much to say in terms of quality, in my books. My rule of thumb is try to avoid things that might be 'unpopular'. Cast quality is even variable in individual casts. You might get unlucky and get a horribly fucked up infantry model but the other 9 infantry are perfect.

Another rule of thumb is the more large, flat, thin panels a piece has, the more likely it is to come warped. This was true for my tanks, but the warping took all of 5 minutes to fix, so worth it. Also, models with highly intricate, small details and thin areas can be problematic. Something like thin lasguns or antenna accessories might just snap off in shipping.

Honestly, at the end of the day, I don't care about quality too much. For the money, Z is magical. I mean, if you're concerned about the large flat bits of a Land Raider order being warped or its accessories snapping off... then order 3! 3 Z Land Raiders cost the same as 1 Games Workshop one. You get uh. More Land Raiders and if one is messed up, you still get two. This is an extreme example. What I'm saying is when you order recasts, you compromise quality for price by default. Don't expect good quality. If you're lucky, you might get Games Workshop quality on one or two things, but don't count on it and don't sweat it.

Get a good set of tools, knives, clippers, sanding things and work carefully. You'll get lots of practice cleaning things up with Z models and you'll get good pretty quickly.