The best party buffing classes are:
Icemage (krit & aspd +15%, cp reg +50%, mspd +25% 1/3 of time & atk +10%, kritdmg +20% > 1/2 of time)
Firemage (atk + 4% 100% of time)
Bard (Various melodies + atk +10% 2/3 of time)
Earthguard (atk +3%, piercing +6% 100% of time)
Zerk (atk +10% 1/6 of time)
Reaper (crit +10%, aspd +10% 1/6 of time)
Classes with no buffs:
Duelist, Assassin, Stormguard, Tormentor, Hunter, Templar.
Crusader and Protector have good buffs but not as good as the best classes. (aspd +7% 1/3 of time & atk +10% 1/12 of time)
Good debuffs are done by
Templar (dmg +9% 100% of time if played correctly)
Firemage (dmg +10% depends)
Icemage (krit dmg +4-20% depends)
Bard (dmg +10% 100% of time)
Stormguard (dmg +15% 1/8 of time)
not mentioning the -10% krit eva -10% def that various classes can debuff.
Paladin is always a good choice, tank is always well apprectiated in raids and crusader does very good dps.
Hope i could help, feel free to ask.