I know this is an exact duplicate, but a year has gone by and Scala seems to be a fast moving thing, so I figure it might be acceptable to ask again:
What is the bes
Right now, IntelliJ's IDEA. And one big difference from now to a year ago is that a free, open source version of IDEA is available.
Personally, I use IDEA CE 9.0.1, but leave compilation&testing to SBT, which I keep running on another window, with cc or ~test.
cc
~test