LuaJIT is like 5.1 with some enhancements that don't break anything and with some optionally enabled ones that can break stuff:
http://luajit.org/extensions.htmlI personally sticked to 5.1 after learning 5.2 since they are so similar (changes between Lua versions are minor and/or arcane, like env change and fix of hashing algorithm vulnerability between 5.1 and 5.2, etc.) and since anything that you do in 5.1 is 100% doable with LuaJIT if I have problem with speed of Lua code itself.
5.1 was also around the longest. It came in 2006 and 5.2 only at the end of 2011 and 5.3 at the start of 2015. That, incremental GC (5.0 had a stop-the-world collector) and having that wonderful JIT (and for a while - Havok company had its own implementation of 5.1 too) is what made it such an important version, especially for games. I'm writing a paper about Lua 5.1 right now actually so I'm kinda familiar with its internals, design and history.
5.3 disappointed me with the addition of int number "subtype" (I liked that Lua only had floats), binary ops (I don't care, I'm not writing anything that crunches ints in Lua, I'd write a C function for anything like that, I see Lua as embedded language, not a language to do everything in, like Python) and so on.
This forum even has Lua code highlighting if you use that.
And speaking of Lua, it inspired another language called Squirrel, Source engine from Valve (Half Life 2, etc.) uses it. There is also the AngelScript that is unique because it's statically typed and looks like C++ (to some people that's a plus, to me it's not, if I wanted a static 'script' I'd use something else, the point of scripting to me is to be dynamically typed). I know Penumbra 1 used AngelScript, maybe next games from Frictional did too. I think it's really easy to use too. And they both look more like C/C++ (to me that's a downside because I want to never ever even think of the other language when coding in either C/C++ or my scripting one but many people like the syntax similarity).
I'm personally sticking to my Lua 5.1, it's like the king of small languages. And I like the syntax. I just hope a groundbreaking 6.0 doesn't happen soon because that'd split the user base so badly compared to just 5.x versions for which you can easily write small bits of code in C and Lua that are compatible with all three.
I also like that it's designed by three guys who clearly know what they are doing and that it was a language to solve real life practical problems from day one (or even day zero or day minus 100 if you consider the predecessors). It just sits in a really sweet spot, it's not a language that you could consider your main skill because it's too small, but in addition to C or when scripting (I've seen designer jobs at Klei, the makers of Shank and Don't Starve that were basically: experienced designer + Lua scripting skills) it's just so damn good.