Which is more important in a musical? is the acting or singing?

I'd venture to say that of the two, singing is far more important. At the same time, if a performer can't act well enough to be convincing then the show is going to be a dud. The fact of the matter is that there are a host of multi-talented performers out there not only can sing and act good enough for the Broadway stage but can also dance.

It really depends on the material.  In a show like "Ragtime", the singing needs to

be paramount.   In something lighter, say "Spamalot", the dancing  (and the

non-dancing King Arthur) needs to be emphasized.

The singing is far more important than the acting in a musical, for me. But every member of every audience has their own opinion.

I've thoroughly enjoyed musicals where the musical performance was great and that acting so-so, with muffed lines, poor diction, etc.

I prefer the story to the songs. The songs in a musical sound so contrived.

Now, if it was a concert or an opera, where most of the dialogue is sung, the singing would be more important.