Right now Jamup/Bias, Ampkit, and Tone Stack are the best amp simulators for iOS in my opinion. Each are outstanding in their own right and each bring something special to the table. Their are a few other apps around that sound darn good as well (Amplitube Hendrix, Flying Haggis, etc.) But it is hard to go wrong with the top three I have listed above.
Bias on its own sounds terrific. The metal amps are killer in particular to my ears. Combined with Jam-up, Bias becomes almost an unstoppable monster. Why? Not every guitar will sound good with certain amps. With Bias you can dive in deep and tweak or create an amp or several amps that will sound good with the guitar you are using. When you are done tweaking you can load your custom amps into Jam-up to take advantage of all the additional features Jam-up offers.
In summary, different people will like the sound of a particular product over another time an again. However, with Bias you can change the sound of the product in ways far beyond any guitar amp simulator I have found on iOS. It really does feel like having a custom amp workshop in your pocket. In this regard, it is hard to imagine that if someone spends some time with Bias, they will be unable to dial in a quality amp sound. It is that good!