First I was incorrect- there was a different host for the US version of The Weakest Link.
Second.. Ken Jennings didn't "do too well"? WTF? he won 74 games in a row.. how is that not doing well? Even the smartest person will make mistakes sometimes and not know EVERYTHING. He was quicker on the button too which was part of his strategy.
As for Are you Smarter than a 5th grader? show- yes they did have celebrities show up on that show from time to time. I've watched some of the reruns.
Every game is different. Survivor is definitely a lot of backstabbing (Same with Big Brother euugh). But with Weakest Link- to me it's more strategy than anything. You need a way to at least determine a winner... going round robin with 9 players for a half an hour is boooooring.
Matter of taste what people think is the 'easiest' and the 'hardest'. I like Jeopardy and oftentimes I do get an answer that none of the contestants get 
. However my all time favorite game show will always be Match Game.
