It's difficult for me to compare and rank Dostoevsky's major novels, but I'd probably consider Demons to be the most underrated...because it seems to be seldom mentioned, but is truly as great as any of the others imo.
And the striking relevance to our world today makes it all the more significant. Human nature just keeps repeating the same patterns.
It's the 4th that I've gotten to, thus far:
I've read
The Brothers Karamazov (several times), as well as
Crime and Punishment and
Notes from the Underground.
Brothers is among my favorite books of all time, and I intend to read it every so often for the rest of my life.
C&P was very valuable, but a one time read. I wouldn't want to read it again, but I'm glad that I did once.
Underground was far less valuable to me. I don't mind that I read it, but I'm not highly recommending it either.
Now I'm into
Demons and although I havent finished it yet, it's shaping up to give
C&P a run for its money as #2 for me.
I also intend to read
The Idiot, but from reviews I think that it may not seem much more valuable to me than
Underground.