I think all of the amps that I really love in the Axe are "underrated" models.
Splawn Quickrod - It's the least Marshall sounding Marshall clone that I've ever heard, but I love it. I run it in 2nd gear with a clean TS808 in front. You need to really crank the treble and presence to overcompensate for the darkness, but once you do, it sits in the mix beautifully and, to my ears, sounds clearer and more percussive than other high-gain amps.
Splawn Nitro - I started playing with this after I fell in love with the Quickrod. It has so much bass on tap that I think most people disregard it as boomy or flubby. I cut almost all of the bass and depth, running them at 1-2 with the bass cut on. Then, suddenly, you get this beautifully aggressive lower-midrange amp that sounds great in stereo with the Quickrod.
Fuchs ODS - Sort of in the same vein as the Splawn, people expect this amp to be a Dumble clone, but it can do much more. It has more mids and a shitload more gain than a Dumble. With the gain turned up, and a clean TS808 in front, it gives me the most searing high-gain lead tones that I've been able to coax out of the Axe, and it sits in the mix beautifully.