They all have strengths and weaknesses.
Guitar Rig is great in so many ways. The interface is clear and easy to navigate. The routings and modulations are ridiculously flexible. There are a lot of great effects. You can get under the hood to unlock a bunch of options (such as how many stages your phase shifter has, or adjust the bias of your amp sim).
It's shortcoming is that the amp simulations are mediocre.
S-Gear has great amp sounds, but a limited selection. Their amps are "inspired by", not "branded", so if you have to have a model of a specific amp you might not like it. Also, if you are a Vox fan, it's not the one for you (yet). It does not have a large selection of effects either, but the ones they included sound good.
Amplitube 3 has really good cabinet simulation (most other use cabinet impulses). It has a huge selection of amps and effects, most of which are officially licensed.
However, Amplitube has a few issues. First, with the "photorealistic" rendering of the effects, it can sometimes be hard to see where the knobs are set because of glare from the simulated lighting. Also, I prefer to be able to see everything on one page, and with it you have stompboxes on one page, amps on another, cabs on another, post-effects on another. In addition, I'm not crazy about the reverbs.