If we're really talking MUST have guitar software, then either Tonex or NAM is all you need, since those are simply the most realistic soundwise. Modulation effects such as delay, pitch shifting or reverb can be done by non-guitar software.

 

No software that I ever have used in front of a guitar can for me replace an amp pushing air...though the Strymon Iridium for whatever reason worked well enough for a time until it didn't please sonically any more. What I have though enjoyed while using it for that time was finding it's sweet spot before breakup and rolling my guitar's volume knob to keep it dialed in there or digging into the strings when I wanted to growl. Now I just use my little AC4TV and pedals.