AS3 is a pretty underrated language. It was ridiculously fun to make stuff with it, and if you bought the official Flash Builder IDE (which was Eclipse based), you had decent autocomplete and everything.

The runtime definitely needed to be improved, but I feel Flash gets a bit more hate than it deserves. By the tail end, there was even decent 3D graphics support, and CrossBridge was a pretty cool predecessor to Emscripten that allowed you to convert C++ programs into SWF stuff (IIRC an early version had the Doom engine ported over).