It's a bit like drum software - people have their favourites.
S-Gear - great sound but doesn't have the 300 amps and 400 effects like the others.
Guitar Rig - Not quite marmite but if you like, you'll love it. GR5 is an improvement by all accounts.
POD Farm - Line6 can have a Line6 sound to it. Lot's of sounds and flexibility.
Amplitube3 - Has a lot of choices and flexibility.
TH-2 - ditto
I've heard them all used brilliantly so you need to try and buy. I'd say most people end up buying most of them that are available. There's always a sale on them at some point and that's probably the best time to buy each one.