Reason is what I recommend (www.propellerheads.se). It's advantage for learning is that it's a closed environment, so you only learn the basic building block components of production (which is enough to grapple with for a long time). As another bonus of it being closed, you can deconstruct project files from:
www.reasonstation.net
...and see how they tick. No other host can offer this, as far as I know.

Everyone's going to recommend what they learnt on, or what they're using now, but as a learning tool I think reason has a significant advantage over VST hosts because it's limited. That means you can learn directly from other people's project files, and it won't drown you in the maze of options and gimmickry that plugins offer.