most -the most
The thing I like most is
The thing I like the most is

Are both correct?

And , must they be followed by -ing, to infinitivr or bare infinitive?

The thing i like most is to sleep/ sleeping/ sleep

Thank you

Question

most -the most
The thing I like most is 
The thing I like the most is

Are both correct?

And , must they be followed by -ing, to infinitivr or bare infinitive?

The thing i like most is to sleep/ sleeping/ sleep

Thank you

Answer

"The most" refers to a thing ("the" for nouns, remember!)

"Most" by itself refers to an action. Here, it's an adverb. (Actually, I think strictly it should be "best", but I'm happy to be corrected.)