I'm a very, very forgiving modeller quality-wise - I'd invariably rather spend £3 on an old second-hand kit and spend
time detailing it than £20 on the latest state-of-the art Tamiya/Trumpeter/Dragon etc. kit of the same subject. And I'm
happy to build grotty old ex-Aurora/Merit SMER WW1 kits or old 60s Airfix kits if they're the only commonly-available kit for their subject.
However, I don't think even I can think of much good to say about Starfix and Kitech/Zhengdefu!
Starfix 1/48 kits are badly underscale (closer to 1/55), have rivets worse than any old Airfix kit, and minimal detail, and are all of common subjects - unlike SMER's WW1 planes for example, you can easily find a vastly better Spitfire, Bf109 etc. for only a few pounds/dollars more. Easily the worst kits I've seen by a long shot, especially considering that they don't have the saving grace of being of a rare subject, or the excuse of simply being old and outdated.
Kitech/Zhengdefu are not nearly as universally bad as Starfix. If you can get over the generic wheels/tracks and the fact that the scale (supposedly 1/48) varies between 1/39 and 1/53 depending on the subject, some of their kits are reasonably crisply-moulded and detailed (Leopard 2, Type 90, T-80, BTR-80, for example) and many are of unusual subjects.
On the other hand, some of their kits are -awful- in terms of moulding and detail - the Abrams and AMX-30, for example. (I wouldn't recommend the Abrams to anyone, regardless of cost; at least the AMX-30 has the one redeeming feature of being the only kit of its subject!)
If you can pick them up second-hand for a few pounds, the Kitech range can be fun to build, but otherwise I'd definitely
stay well clear, especially if you're not on a limited budget.
Most of the other manufacturers mentioned here, I would not include on a list of worst manufacturers, either because their
worst kits are generally very old (Airfix, Revell, Monogram etc.) or they have improved a lot more recently (Trumpeter, Hobbycraft). Others I can't comment on simply because I haven't built any of their kits - SKIF for example.
I wouldn't really include SMER either, at least not their 1/48 kits - though most of their kits are ancient and crude Aurora, Merit and Artiplast re-issues, they are -very- cheap and are mostly of subjects not available elsewhere. They do a very good job IMHO of making these kits available readily and cheaply to modellers like me who are prepared to put in the time and effort to finish them to a good standard. If it weren't for SMER I certainly wouldn't have my collection of 1/48 WW1 aircraft, 1930s RAF biplanes, and WW2 Italian aircraft!
FROG/Novo would probably be the third on my list, but, like SMER, they have made many, many kits of subjects not available anywhere else!