HM is a type system first, and that means is a logic system. HM is a logic that is deliberately kept relatively weak in order to allow full inference. Scala is not HM and never has been - it's much, much more sophisticated and so Scala uses a form of type inference not based on HM. Haskell '98 is HM plus a tiny bit (and yes that tiny bit does mean that the occasional type annotation is required). GHC Haskell is HM plus a ton more and so there are a lot more GHC Haskell programs that need some type annotation. The people behind GHC Haskell have done a good job of carefully adding type system features that don't totally break the model, though, so you rarely have to annotate much unless you love playing in advanced type land.
I would hesitate before saying GHC Haskell is more sophisticated "by far". It's not at all obvious that that is so. I'd want a type theorist to spend some time with the two core logics to determine which one subsumes what parts of the other before making that call.