The new layers.library and MCPs solid window move/scale patch.
Superfast with 20 or more overlapping windows on RTG.
With the original Commo lib it was crawling even in WinUAE.
MCP itself... well you can hate it or love it but there is not much in between imho.
And I couldn't live without Freewheel and MagicMenu anymore.
AfA OS(only in emu) just for a more modern look. It has some flaws when used with Scalos but I like it much more than the Birdie/VP combo on my real 1200.
It's a real bummer that it needs at least a 16bit screen to work and a fast CPU, of course.