Just buy the cheapest one (2003?). For a learning tool, it will suffice. If you wanna buy the best one, VX Ace is good if you just want to use what is in it. If you want the best one, MZ. The problem with MZ is that what makes it good are the plugins, where the good ones cost more money. For pros, its essential (fine, recomended). But to start, I wouldnt bother and just buy the cheapest one. You can also wait for them to go on sale on steam or humble bundle.