AmigaOS? pfft. Sorry.
On the other hand, I DO use Workbench 1.3.3 the most.

Well, on the A500. I actually prefer it.
On the V4000 (UAE pretending to be an A4000/040)....
I use 3.9. I'd use 3.1, but no disks

And which do I prefer? Honestly... Workbench 1.3. The color scheme is easier on the eyes, I like the icons, and unlike 2.x/3.x, the icons are always proportional. (But then again, afaik, you're stuck with one screenmode in Workbench, so go figure.) I've never seen much reason to upgrade. 3.9 is used for little more than preparing disk images to send over...
PS: My first thought when looking at Tone007's avatar: "Seth Green owns an Amiga???" Looks like I'm not the only one. xD