Let me rephrase you question thusly: Who was the least phenomenally brilliant and awesome world champ? I guess I would say Euwe or Smyslov.
Reb wrote:
There is always much discussion over who was the strongest of the world chess champions and each of us has our own favorites. I would like to ask who do you think was the weakest champion? I will first cast my own vote for Max Euwe.
Topalov, Ponomariov, Kasimdzhanov and Khalifman shouldn't even be in the discussion because they weren't "real" world champions, even though Topalov is a very strong player in his own right and is hanging around 2800 FIDE, and therefore would be stronger than many real world champions. However the weakest real world champions would have to be Steinitz, Euwe and Smyslov.
People that are saying Anand or Carlsen or Kramnik just because they're in the modern era need to realize that you can't speculate how good the old guys would have played if they were alive now and had computers. As of right now, Anand, and Carlsen/Kramnik especially, usually play nearly flawless chess, especially middlegame downwards. Kramnik is starting to slow down, but closer to his peak, and in recent years even, he has been impeccable. And well, Carlsen is the highest rated player of all time and untouchable by anyone not named Fabiano Caruana.