The latest "2.x" version of gcc/g++ is 2.7.2, released November 26, 1995. The latest version of libg++ is 2.7.1, released November 12, 1995.
Don't use 2.5.x, with x less than 5, for C++ code; there were some serious bugs that didn't have easy workarounds. 2.5.8 is the most solid 2.5.x release. 2.6.3 is the most solid 2.6.x release.
For some non-Unix platforms, the latest port of gcc may be an earlier version (2.5.8, say). You'll need to use a version of libg++ that has the same first two digits as the compiler version, e.g. use libg++ 2.5.x (for the latest x you can find) with gcc version 2.5.8.
The latest "1.x" version of gcc is 1.42, and the latest "1.x" version of g++ is 1.42.0. While gcc 1.42 is quite usable for C programs, I recommend against using g++ 1.x except in special circumstances (and I can't think of any such circumstances).