First, you have to define toughest. Now, overall, \"I\'ve Got a Secret\" seems to be easy - be a friendly host, know which panelist you\'re starting with. But add in the stunts the host did, some of them with a legitimate amount of risk, and that gets very tough.
Granted, that\'s an exceptional way of looking at it. I think the goal here is \"which show required the most overall work by the host?\" Given that qualification, yes, TPIR would seem to be tops - if you are new at it, or Drew at it. I\'m sure it was tougher for Barker in the first few years, but after he got the games down, he was able to concentrate on interactions with the contestants - something he put on autopilot in the later years, and something Carey is inconsistent at. I agree Brady might work better on TPIR - he\'s sharp enough to pick things up and works very well with the contestants, For LMAD, you have to have that patina of the sharpie salesman, something Monty Hall did very, VERY well.
You could also say that the games that are toughest for the host are those that require the host to take an active part in the game, as Hall/Brady would do in trying to convince and confuse the contestant. Consider \"The Newlywed Game\" - the host is not only there to ask the questions, but perhaps nudge the contestants into embarassing themselves, something Bob Eubanks did ... you get the idea.
Now, that doesn\'t mean a host who just stands/sits there and asks questions doesn\'t have a tough job and a key role in interacting with the contestants/panelists. That\'s what made Cullen the master - teasing the contestants, joking about the game, and generally lending a friendly aura. Or \"What\'s My Line?\" Being younger when it first ran, and not as educated, I never realized how much humor John Daly\'s obtuse \"clarifications\" added to the show.
And then there\'s knowing when to step out - Peter Marshall running the Squares, not as a funny man but as Mr. Setup.
Each show required unique talents - IGAS to play along with stunts, WML to be the wise misleader, TPIR/LMAD to keep a contestant who is about to lose his/her water somewhat focused on the game but still excited and do so with a variety of approaches, \"Malcolm\" to interact with a puppet.
Having thus confused the issue, I withdraw. (Now if you ask me what is the toughest job for a sportscaster, I will nail that one - horse racing.)