Favorite Guitar sims

Atomic Amplifire is very good for $599.
For sims I prefer S-Gear 2.52, and Amplitube 4...I also have Revalver 4.

If you've got SPlat I'd give the included TH3 a try first before spending any additional $$.   Oh, and the CA-X amp sims (under FX chains).  Between those you have some good options.   But if you then find that they're still not giving you quite what you're looking for then you can demo some 3rd party plugins (like S-GEAR). 
 

GuitarRig is my first choice.  The improved sounds of amplitube didn't offset the difference in flexibility of GuitarRig.

Over time I've collected a few sims in bundles or during sales.  GR5, AT4, Waves GTR, TH2/3.
 
I've used GR5 a lot.  Then I went to GTR for a while and now AmpliTube with the Mesa bundle.
 
I also have a Laney IRT Studio amp cabled up for direct recording.  I really like this amp feeding cab/mic sims in one of the mentioned sims.  Also in hardware, the BOSS GT-100/001 sound is very good.  I have the -001 (desktop) and like it a lot as well.
 
IMO, they all have pros/cons.  GR5 is probably the most complete, but is getting dated.  GTR was a total surprise to me when I bought Waves Gold.  IMO, GTR is a real gem and is also quite complete.  I can't get past the clumsy interface of TH3, although what little I've tried has sounded really good to me.  AmpliTube 4 Mesas are in my current project.
 
I think it would be difficult to make a really bad choice ;)  And don't rule out configs like going direct with GR5 in front for effects followed by another for the amp.
 
Speaking of which ... going all software lets you record direct and then waste hours and hours looping and tweaking your sim to get things just right -
 
Watch the Deals forum ...

cclarry
My Hughes & Kettner Grandmeister has XLR out with Redbox built in
for direct recording...that's a pretty sweet feature....

Those H&Ks have been tempting me forever.  If the 18 had reverb I would prolly own one.
 
My Laney has built-in direct support as well.  I have a Red Box 5 I use with my Bugera V22 from time to time.  The thing I like about the Laney IRT is that the direct out has switchable emulation, which allows me to feed ITB cab/mic sims.
 
I use the RB5 with my HT5RH too.  It has direct out, but it's basically just the pre routed thru emulation.  Sounds good though.
 
So many options these days.

S-Gear, PositiveGrid both BIAS FX Pro and BIAS AMP Pro.
 
I got rid of my Line 6 HD500X and rarely use my Eleven Rack boxes anymore.  I would love a Helix but I can't justify the cost.

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.
 
 
 
 

I agree with most of the comments above, I think that you are best to start with the type of amp/ sound that you want  and work from there. Each developer seems to have strengths and weaknesses.
 
For me Bias FX is now my go to because it has the best Vox sound in my opinion. Unfortunately Vox does not seem to get much love from amp sim developers.
 
For fender I would agree that Amplitude or S-gear are the best on that front.
Amplitube are also great for the orange tones.
 
I don't go for heavy sounds so cannot comment on those.
 
The other big part of getting the best sound for me has been getting some great Impulse responses to match with the amp sim.
I have a selection of Redwire IR's and use them with great effect.

Crowned One
For me Bias FX is now my go to because it has the best Vox sound in my opinion. Unfortunately Vox does not seem to get much love from amp sim developers.

 
Bias totally markets to the mega gain crowd so heavily that I started to wonder if they were trying to distract people from their clean/midrange sounds not being any good.  I may have to check them out.

Another vote for S-Gear here. Fender and Marshall sounds are it's specialty ;-)

Thanks for all the comments, I leanerned some things and had some things confirmed. 
 
I should have mentioned that I have TH3 full (recent Cake sale), and Amplitube 4 (iRig sale).
 
TH3 has a ton of stuff, but many of the amps are high gain weirdos that I am not familiar with.  For some reason most of the presets have a Ross compressorset to an unusable setting. (Thanks YouTube).  I find the interface on TH3 to be a bit Byzantine.  I have to give a shout out to Overloud including a DC2, one of my favorite pedals. 
 
I got Amplitube 4 Deluxe on the current sale for $100 with an interface thrown in.  The Marshalls, having been most recently updated, seem to be pretty good.  I had some IK points to get the Vibroverb amp and cab and like it as well. 
 
I also like the user interface, it seems a bit more immediate. 
 
I also have Waves GT3, and haven't even given it a spin.  I have some work to do. 
 
I suppose part of the challenge with sims is learning what each does well.

Sidroe
Amplitube 4 and TH2 and 3 for in the box. Threatening to get BIAS. Quick and dirty live or sessions finds my POD HD500X direct to the board or usb to the computer with the Spdif set for clean to a separate track. Making the jump to Helix shortly.

You're going to like using Helix.
The UI is amazingly simple/powerful
The latest Marshall and Mesa Mk IV models are really good
 
If/when you pickup Helix, go here and download my (free) Friedman 2x12 cab IRs.
http://www.studiocat.com/temp/Friedman_2x12_cab.zip
With numerous mics/positions... the collection works well for nearly anything.
 

No mention of TSE X50? SHAME.
 
Really nice 5150 simulation, and for $70 it's impossible to go wrong.

Although this is not as full featured as the bigger amp sims out there, Brainworx RockRack Pro is very good (IMO) for rock/metal tones straight out of the box......of course, YMMV. Definitely was well worth the $29 deal they recently had......never would I even consider it at the current price of $249.