1 - pick your favorite game ... Warcraft for example...
2 - create some custom maps or levels ...
3 - see if anyone like your levels better than the default ones...
4 - see if this is fun for you ....
Next, look for a system that lets you do what you want...
If RTS is your thing, look into any open source engines (there are many FPS engines that are open source and just awesome)
Which ever way you go, you need to push the engine as hard as you can. Break things, just so you can learn how it works
so you can fix it. Everything is complicated. there is an open source JS RPG MMO engine even .... with a level editor...
You do not need to begin from scratch unless you have mastered the engine of choice and wish to do something the engine
does not permit .... At that point, look for any dev system that come closest to what you already learned from the previous engine.
If you use the JS RPG and you have exceeded it's design specs, maybe its time to roll your own in custom JS? or HAX.
Simply asking which is best will only start a meaningless discussion ... I have used everything from MASM, C++, PHP, Prolog, many flavors of basic and FASM
on up to JS ... so far, the only one I truly dislike is Python ... just something about the syntax that just annoys me to no end. In short, focus on something you love
to do... after all, if it is your game, you will be doing it a lot. So, If you love the syntax of Python, and love making your platform MMO game, expect to spend
most of your time in the Python editor... (you can replace Python with any language or IDE and my point is still valid). Pick something that "Just Works" for you.