Despite it being my favorite anime (I'm really not privy to very much), I wouldn't say so. That's a pretty big statement to declare it the "best anime in existence".
I certainly wouldn't call it "under-rated", not that it's over-rated either. It's absolutely massive over in Japan, which is more than enough for me. I do wish that it were more popular in the states, but there were several reasons why it failed.
I'd say that Detective Conan satisfies a certain "itch" I have as a whole, which has kept me attached to it for a very long time, and I still find the new cases interesting.
The quality of the anime isn't exactly great at the moment. Many of the anime original cases are flat out uninteresting, and I really prefer the older animation style. The show has long past its peak, and I definitely prefer the manga as a whole.
Having not watched much anime (mostly just what was on Adult Swim in the past), I can still say there are some that I've enjoyed almost as much as DC, if not more at some times. I love the Ghost in the Shell series, as one example. Having only seen about 10 or so all the way through, I really don't think I'm in a position to label anything as "the best in existence".
Narrowing it down to its genre, I might agree.