DO NOT USE JAVA!
Ok, runescape did quite well with java for online, but if your planning to make an in browser game like runescape you will have to deal with Java's securities. Java applets prevent you from opening a socket connection to any other server than the one hosting the .jar file. So if you are going to host the server for your game, it will also have to run a webserver that host's the jar file. On top of that Java is SLOW. It has little to no graphical hardware acceleration. And you'll have a hard time getting the opensource OpenGL java binding to work with an applet. Its just a disaster waiting to happen. Runescape wrote it's 3D engine far before Runescape was developed I believe it first appeared in a Jagex game known as Vertigo.
Also even though java is run by a virtual machine. Its notorious for being very difficult to sign your applets and get function on other computers. It can be an extreme pain in the rear. I would not advise writting anything the size of a Online RPG in java.
However....
C# is WINDOWS ONLY! You'll most likely be using the .NET Framework. Make sure you have a pro version of Visual Studio so you can create a proper installer for your game, or else it will fail to run on every windows machine.
C++ is probably the most robust and powerful, its multiplatform, its like a solid tank. I'm writing my Pokemon MMO in C# just because I enjoy the .NET framework however if you dont like .NET its probably not the solution. C++ has a multitude of free libraries for 2D and 3D personally my favorite for 3D is Irrlicht3D however Ogre3D and Crystal Space are just as powerful. Ogre has better resource management =).
Notes on other Online RPG related things in programming:
If your planning on unique NPC's and quest's you'll want to invest some time in implementing a Scripting Language. As far as they go, Look into Python, Lua, and GameMonkey all are quite power, Dont write the game in these!!!! Instead write bindings so you can write scripts for your game in these languages and have them control NPC's within your game!
Server's, you'll want to have a reliable host for your server, its gonna need a static IP. You cant have your games losing the address to the server. You probably cant host it from home, You'll need to pay for some hosting elsewhere with proper computers and ISP's.