In EQ as a healer or caster, it was always Clarity. I would usually decline group invites unless an enchanter was present. Spirit of the wolf was always nice but I can live with being slow vs not having any mana to heal.
In WoW before all the buffs were removed its hard to say which buff was best as they were all extremely relevant. If I had to order the buffs in priority though: 1" Mark of the wild (as everyone could benefit from it) 2. Fortitude (as all classes could benefit from more health) 3. Arcane brillance
Using EQ as the example, like Mauv mentioned, Avatar was amazeballs as a melee (it also increased your ATK rating) but I also loved Dead Man Floating because it was one of the few spells that used no reagents, lasted an hour and gave enduring breath, levitate, and see invis as well as PR and DR. SoW was great as a lowbie, but by end game everyone had j-boots, and I would say it is safe to assume there will be an item that grants movement speed in PRotF that will be available to all classes.
Ultimately, stats were usually capped where you needed them capped so I never found Avatar all that important even if I had a Primal Dagger if I wanted to pop it. We all had J Boots so SoW was useless, DMF was basically levitate which was in just about everything anyway from levitate as a spell, to Air Elemental Illusion to levitate clicks. In general, for me it really comes down to what buff works long term and what provides the best in terms of performance while being difficult to obtain from an ulterior means.
For that reason, Wizard O'Keils Radiation reigns supreme.
Joking, it's still Haste and Clarity.
BTW - For those who would stand in a Pure Caster guild in town and wondered what all the 'flame applied' casting noises were. Wizard NPCs or guildmasters would cast this on every NPC within range every 2 minutes. I think they took the spell off the cast list for them later on but the first 2 or so years, my god was it annoying if in the area buying spells for your level.
