What do you think is the main conflict in the novel? Why?

I'll add a slightly different slant and suggest that the central conflict in The Great Gatsby is illusion vs. reality. 

The novel is first and foremost about Gatsby.  It's named for him and it is about him.  When Nick concludes the novel he does so by referring to the subject of recapturing the past, which is what Gatsby tries to do the entire narrative.  That is what the work is about.  I suggest that everything else is secondary. 

The book is above all else a love story.  Gatsby's love for Daisy is what makes it worth reading, the same as Shakespeare's Romeo and Juliet is primarily a love story and that's why it's still popular.  The general population doesn't watch Romeo and Juliet to see what it has to reveal about feuding.  Numerous other issues exist in that tragedy, too, but it is above all else a love story.  Gatsby's story is the same.

Gatsby's view of his relationship, however, is an illusion.  His love for Daisy is everything he thinks it is, but Daisy's love for him is not.  He spends five years of his life chasing an illusion, and he refuses to give up on it even when all rational thought and all evidence suggests that his illusion is false:  of course he does, he's in love.  Society cannot stop Gatsby, neither can money.  Gatsby has money.  If that's all there was to it then Daisy would have gone with him.  Again, Gatsby has money--Daisy wouldn't have to give up a thing as far as money is concerned.  Which man has the nicer shirts?  Gatsby, of course. 

Daisy does not go with Gatsby because the illusion in Gatsby's mind is false.  She refuses to say that she never loved Tom.  She refuses to say that she's been pining for Gatsby all these years.  He asks too much, she tells him. 

The only thing that can stop Gatsby is reality.  Reality trumps illusion in The Great Gatsby.