i just completed a quest where i was sent to an isolated island to find some chick's husband. turns out the island is home to a fort and the fort is used on some hunt dudes for sport tip. i'm locked in this fort with hunters armed to the teeth looking to kill me. things aren't looking good

so i decided to abuse the 'shadow self' (invisiblity for 30 sec, goes away when action {attack, open door/lock, etc.} is taken, illusion school) spell since i'm definitely at unfair odds here. and boy did it ever work. most 'fights' went something like:

me: *casts 'shadow self'*
me: *sneaks up on unsuspecting hunter*
me: *sneak attacks with glass longsword for 6x damage*
foe: ouch!
me: *runs behind object that conceals me and casts 'shadow self' again*
foe: ????? *loses track of me, searches for me for a while, gives up and sheaths weapon*
me: *sneaks up again*

etc, etc.

so i kill off 3 of the hunters this way and finally grab the key which opens the gate back up so i can get the fuck outta there. i make my way back to the entrance and lo and behold, the last hunter, an orc, is waiting for me with his archer buddy. time for 'shadow self' again

i cast it as soon as the orc is done telling me he's gonna kill me, run behind a large column, and hear him say 'he was just here a second ago. where'd he go?' then the good old bearfield instinct kicks in. i run up the 3 flights of steps and jump to an isolated platform where the orc can't get to me and the archer has a tough time shooting me. 8 invisible sneak attack arrow shots later (snipes, if you will), they're both dead and i completed the quest without having to use a single healing potion or healing spell

couple this incident with the fact that you can rob entire castles right under the noses of guards without being seen and it makes pickpocketing so much easier (i've stolen weapons off ppl this way) and imo you've got one of the best spells in the game. i'm sure the best spell will be whatever spell gives you 100% chameleon (invisibility, doesn't go away when action is taken) but that's a long ways off