Flash is great for stuff that runs in the browser, but for server-side heavy lifting, C++ is my go-to tool.
C++'s many flaws and disadvantages have been carefully documented over the years. The language is too big; the compilers produce lousy error messages; the standard template library is too hard to understand; there is no need for a low-level language in this day and age. My biggest gripe with C++ is the sheer number of ways to introduce near-impossible-to-find bugs: using uninitialized memory, freeing memory twice, using a dangling pointer, mixing malloc/free with new/delete, leaks, overruns, dereferencing NULL ... the list goes on and on.