First off, I agree with Amon Amarth. Anything with that blend of heavyness, rhythm, melody and sheer epicness is going to work well, though I'm yet to find another band that adds all four elements by the spade.
In general, less fussy about stuff for weights- anything that gets the blood going is fine, depends on my mood. For cardio, I agree that more straight forward melodeath (i.e. fairly constant time signatures and up tempo rhythm), good metalcore and groovy death stuff works well- add Jungle Rot, Malevolent Creation, Vader (Necropolis has been doing in for me recently), and I can't believe no-one has said Bolt Thrower yet.
Further random additions that have worked well for me lately:
Fear Factory- Mechanize
Immortal- All Shall Fall
Hackneyed- Burn After Reaping
Man Must Die- No Tolerence For Imperfection
Trigger The Bloodshed- The Great Depression
Behemoth in general