what is the best armour?  

Ok, here goes ( remember , there really isn't a BEST as each has it's own benefits in certain situations ).

F2P
Mage-Blue mage or FOG robes
Range-Green dragon
Melea- Full rune ( the god trimmed stuff has the same stats )

P2P
Mage- offensive, Infinity ( mystic is close second )
Mage- defensive, Splitbark or skeletal
Mage- Balanced, lunar

Range-Armandril

Melea, full dragon or Bandos, full granite is close behind.

This does not include 3rd age treasure trail sets which are supposidly the best non degrading armors.

The Barrows sets in each area are very good but they degrade.