Cannibalize! So much fun being able to regulate your mana level by consuming your health. And with Iksar natural regen and a regen spell on you (or even better if you had a Fungi Tunic) you basically had unlimited mana and could solo things that you otherwise could not. Good times!
There are so many unique spells that I have come across over the years, but if I had to pick one or one group of spells It would be the Mage pets from EQ1.
20 years later and they are still some of the most iconic and well-designed pets that I have ever seen. Even as a healer there is just something about bending elements into living forms that I find awesome.
My favorite ability was from the Minstrel in Dark Age of Camelot.
They had an instant cast mezz that could be cast while moving. Long recast timer but I loved that ability. People that would try to race through the fortress to help their realm if we were sieging it I loved to chase and insta-mezz so others in the realm could whomp them. When our group was running around to challenge other groups I liked the insta mezz to save my skin, others in the group that may have had someone on them, or the opposing realm. Naturally, it was a very overpowering ability - I don't believe any other class in any other realm had it.