Too many to choose from. A lot depends on your budget, whether it's a
corporate or personal purchase, the need/desire for tech support, etc.
http://www.8052.com has a list of many offerings at their "links" page
and others at their "8052 store" page.
Personally, I like command-line tools, my own text editor (gvim) instead
of an IDE's integrated editor, and plain ol' makefiles and I don't
particularly want a debugger/emulator so I often go with sdcc.