There's no one best. You can check out this article for some opinions.
Using Solid State Rigs on AM
http://www.eham.net/articles/9842Most of the top-of-the-line DSP HF transceivers can be made to sound very acceptable on AM, and some of the older ones will do so with a little effort.
However you may want to question spending $2000+ on an IC756PRO to reproduce the sound of a $250 Heathkit boatanchor.