Someone's gonna say this...
Gene Rayburn and Break the Bank... Obviously Gene was led to believe that BTB would allow him to do his goofy antics. It had potential... if you watch the first week Gene does joke around wherever he can and he appears to be having some fun, but Kline and Friends later decided that BTB should be more "serious", and in later shows Gene looks bored. It's also obvious that Gene was not briefed about any of the stunts nor where they were on the stage. Either Gene didn't want to be briefed in order to be more spontaneous, or nobody thought that Gene should know these stunts to make the game more smooth. Gene was fired, Joe Farago replaced him. Joe was no better, and dropping the stunts made the show VERY dull.
John Davidson and 100k Pyramid... John had a hard job, trying to fill the shoes of a host that hosted Pyramid off and on for 17 years. John appeared ill at ease sometimes, and repeated himself a LOT (just as Drew Carey did in early TPIR shows). Obviously it's debatable if John could be classified as a GOOD host... I felt he handled HS well, at least for the first two seasons. I think Pyramid was too restrictive for his personality, and for me, he didn't bring the right amount of suspense in the winner's circle.
Ray Combs and Family Challenge... This might not be fair. Ray was obviously going through hard times and he was probably very lucky to get the gig. Ray really gave his all and there were moments where his personality fit in with the show, but knowing what we know now, I wonder if Ray felt deep down inside that he once had a hit show, and now he's hosting some Double Dare/Beat the Clock ripoff that'll never enjoy the success of Feud.