If you just want to create turnbased RPGs in a short amount of time, you can use any one of the RPG Maker Engines. If you want the most recent version RPG Maker VX Ace is the way to go. I think a lot of people would tend to advice RPG Maker XP, because there are way more scripts that get you around certain limitations, but to be honest I'm not a big fan of it ("it" being RPG Maker XP, that is). In doubt you could download the trial versions and see for yourself, which one you like more.
If you want to go more indepth and create games not restricted to RPGs and RPG smilar genres, you might as well go for Game Maker, but it affords a lot of your ability to write and use scripts. If you want a space shooter, you have to write the scripts yourself (or find them on the internet); if you want an RPG you do the same (needless to say it takes longer than simply using one of the Maker engines).
There's also Unity 3D which is basically like Game Maker, but in 3D.