What is the comparative and superlative form of doubtful?

Likely, likelier, likeliest. You may also use the Frenchified forms, more likely and most likely. All English adjectives form the comparative in -er. Some snooty people prefer the French form for certain imported or polysyllabic adjectives, thinking that the native English comparatives "sound wrong." But even they must agree that all English adjectives ending in -y take -ier in the comparative.