Which ones to download

Hello

I used to play a lot on molten before Hermes happened, after that I got some characters made but stopped playing.
Now I want to play again, but I am confused which ones to download.

There are two options named mop (old) and wotlk (old)
I want to download all 3 xpacs, and also be able to play on all realms.

I just need an answer on which 3 to download

Thanks :)