I think that’s a difficult question given that you can only win with the tools you have at your disposal. For example, would Sebastian Vettel have won 3 WDC’s already were he driving for, say, Force India? Probably not. The Red Bull is the best car, but the driver still had to go out and win. Does that make him a bad World Champion because he has the best car? Of course not.
In terms of your question, I’d say we have to rephrase it to discuss which World Champion was a “one-hit wonder”, for want of a better phrase!
In which case you’d have to single out drivers like Denny Hulme, Phil Hill and Keke Rosberg. All had one very good season, but failed to impress in their others.