Hi ace and welcome,
It pretty much comes to what you want to do. For games I would go for steem. If however you want to run applications I would suggest ARANYM. It can be found here aranym.atari.org . Also worth checking is the pack3d at http://ebmodel3.atari.org/Pack_3D/
For games - Steem. For serious applications "Gemulator" works really well.
Aranym whilst very promising is too much hassle to set up to work reliably with many atari apps. It offers some amazing features for graphics apps and applications in general when the compatibility issues have been sorted out.
For Linux and BSD users the answer has to be Hatari.
Best opensource Atari ST emulator available - and for the WinSTon lovers out there, Hatari is a direct descendant, even if it was undergoing excessive rewrites in the past few years!
years ago i bought STemulator. Latest Version 1.68. It is very very fast and it handels TT-Ram as well, up to 128MB and 14MB ST RAM. It can run most applications with high resolution and even native PC resolution: (e.g. 256 color and more at 1024x768). It has many features, but it's not free. And do not expect to run games on it! But it's great for apps.
Magic PC is nice although.