Magic, because it is (well, until Oblivion) the most diverse set of actions in the game. There are so many combinations that form from your own ideas, rather than pre-made combos. They're all player generated. For example, one could do the classic spell absorption + summon ancestral ghost in order to regenerate vast amounts of mana. Another fun one is Mark on-target and a Recall spell in order to blink around.
Then, of course, there are simply the awesome spells of might that one can create. My brother had one, "Army of the Dead", which summoned a ghost, a skeleton, one of every atronach, a dremora, a daedroth, etc. Pretty much one of everything. He had to make sure to look at the ground when he used it because it sometimes locked up the game (we played on the Xbox). It was a very powerful spell.
Then there was effects that weren't supposed to happen, but were awesome regardless, such as the "infinite summon" glitch were you did soultrap for 1 second on target, and summon whatever on target. Cast at your feet, this would create a summon that lasted forever (until it died). You could create armies with this effect, which was really fun as well.
Hopefully some of our (mainly Rhekarid's
) ideas end up in TES V. I really really want magic to be as dynamic and awesome as the magic in pen and paper role playing games.