This also depends on what your interests are. For example, my primary interests are in programming languages, functional programming, type theory and so on. My highest voted comments have mostly been fairly long ones in these topics.
Moreover, most of them were pretty constructive: things like explaining how to generalize pattern-matching, how typeclasses are useful, what FRP is or even just how your editor can display Haskell syntax with Unicode. (It's like an awesome version of syntax highlighting.)
Admittedly, my single highest-scoring post was a bit less constructive: a rant about Go. However, in my defense, it was a long rant with substantial details and examples. And, honestly, I'd be extremely happy if it saved some people from learning Go and got them to consider Haskell or OCaml instead.