I'm surprised I'm the first in this poll to say Nightblade. Actually my favorite character uses a custom class similar to the Nightblade, but much more in keeping with Morrowind's version of Nightblades - substituting Illusion and Sneak for Light Armor and Restoration (like an Agent), and Mysticism for Destruction:
NightshadeNightshades are thieves and covert agents who use magics for concealment, infiltration, and espionage. Swift, resourceful, and nearly invisible when in shadows, they avoid detection by stealth, and rely on mobility and cunning to evade capture.
Specialization: Magic
Attributes: Intelligence, Speed
Skills: Acrobatics, Alteration, Athletics, Blade, Illusion, Mysticism, Sneak
In gameplay, I largely avoid combat with this class, instead utilizing Invisibility spells, speed and acrobatics to bypass most obstacles unless I intend to steal something or otherwise interact with the environment. In those cases, I either use a combination of sneaking and Chameleon spells to remain undetected (while going through doors, opening chests, executing sneak attacks with poisoned short blades, etc), distract large groups of enemies with Frenzy or Command spells while I snoop around, or clear the area of hostiles altogether with Demoralize spells and then help myself to their treasures. Or sometimes I'll use Calm spells to stall enemies until I'm done stealing, then disappear and move on. Charm spells are also useful when persuading NPC's or getting good deals on merchandise. For most locks, I rely on Open spells instead of lockpicking unless the lock's difficulty exceeds my spell capabilities, in which case I'll resort to picking it (which isn't really that difficult, even with very little skill in Security). Pretty much the only other Alteration spells I use are Water Walking and Water Breathing, which allow me to take shortcuts through dangerous areas or escape pursuit without being attacked by slaughterfish. Mysticism becomes useful mostly for its Detect Life and Telekinesis spells, which warn me of nearby enemies and allow me to spring traps on them from a distance.
Since I never wear any armor, my spell effectiveness is always at 100%, so my spells are pretty reliable even against opponents of much higher levels than me. If I absolutely must kill something or someone for some reason, I typically use Frenzy or Command spells to get others nearby to whittle them down for me, and then finish them off with sneak attacks, sometimes employing poisons as well. If no one else is around to assist, I just use Command spells on my target to confuse them while I perform repeated sneak attacks until they're finished. All in all, playing a Nightshade has proven to be both painless and profitable. I've found that the vast bulk of other skills (Destruction, Restoration, Conjuration, Marksman, Light Armor, Speechcraft, Blunt, Hand to Hand, Heavy Armor, Block, and Armorer) are highly unnecessary with this build and tend to remain completely unused, while the remaining few (Alchemy, Security, and Mercantile) become only marginally useful and still not really necessary in the long run. Even most of my own majors (Acrobatics, Athletics, Blade, Alteration, and Mysticism) are of limited importance, since Illusion and Sneak carry me through virtually every situation I encounter. I highly recommend trying it.