I’m currently using Amber Smalltalk (it’s for front-end web development). In the past, I’ve used Squeak and I’ve played with Pharo.
Pharo is probably the best Smalltalk to use at the present time. It’s very actively being developed. It has, I believe, the biggest user group.
You might want to check out Dolphin Smalltalk (it’s Windows-only, though). It has a reputation for having the nicest IDE implementation.
If you’re looking at Smalltalk professionally, you probably won’t do better than Cincom Smalltalk (VisualWorks).