As some people already said it depends on the episodes. I´d say Reese can be really funny. Though his stupidity sometimes is annoying it more often just cracks me up.
I think he´s sometimes even funnier when he isn´t THAT stupid and just reacts to people´s behaviour. Like when Craig told him how great their date is and Reese went "Can we stop calling this a date?" And the look on his face when Craig started singing was just classic.
Malcolm can also be funny. Though he isn´t as funny as most other characters on the show (in my opinion) there were also some times when he really made me laugh.
Dewey used to be funny, but I don´t think he still is. When he was that little weirdo he really could be hilarious, but I can´t think of a funny line or thing he did since the beginning of the 5th season.
Hal can be really funny when he doesn´t go too crazy. When he really freaks out it gets a little annoying, but when he´s just a little crazy (like in Bowling) he can be hilarious.
Francis is hysterical when he battles his grandma. I love all the episodes where they are fighting. Francis and Ida is just the greatest combination ever.
Lois isn´t that funny. Without her the show wouldn´t be definitely the same, but Lois herself isn´t that funny in my opinion.
So I would say everyone is funny in his own way. But if I had to say one, I´d say Reese and Hal made me laugh the most.