What is the best Opensource FSM compiler which can generate c++ code ?
I don't know about the best, but you could look at Ragel:
Ragel compiles executable finite state machines from regular languages. Ragel targets C, C++, Objective-C, D, Java and Ruby. Ragel state machines can not only recognize byte sequences as regular expression machines do, but can also execute code at arbitrary points in the recognition of a regular language.