I like using Lua. It is very easy to understand and very flexible. You can make arrays work the way you want them to (doesn't use arrays, uses tables) and you can make classes work the way you want them to (no object oriented programming, so no classes, but you can fake it easily).

I like Lua since the simple nature of it forces you to think carefully what to do with scripts and what to do with c/c++ for example. as mentioned Lua tables are very flexible and pretty fast so you can do a lot of crazy things with them.

In my current project I do all the per frame update stuff with c++ and Lua just take care of callbacks and define objects

LUA as well :) It's easy, small and fast. And most of all, it makes me feel like its just a small addition to the rest of my code and not some bloated monster that wants to take over and dominate everything.