Pharo is the closest thing to a de facto standard. It has a huge following, perhaps even the biggest of all Smalltalk users. It's the reference implementation for Seaside, the best of the Smalltalk web frameworks. It's the reference implementation for Amber, the best of the client-side web tools. I see little reason to consider anything else.
The best part of Pharo is that you get everything from the ground up: VM code, image objects, tools, all is accessible and can be shaped to one's will.
Try to do that with Eclipse or IntelliJ's tools!
This is making one reach an understanding of the computing platform which is very beneficial in terms of mastering one's craft.
