Hmm...two favorite or two most useful? My two favorite are probably Blizzard because it's freaking cool and works as great crowd control, and any ward spell just cause I think they're a freaking awesome addition to the series. As for most useful/practical, I would say firebolt (low cost and does decent damage even at hire levels, even though you sometimes have to spam it) and any summoning, probably daedra lord, on account of it gets enemies off of your back while still doing damage.

Having two dead thralls of Cicero and a basic city Guard, in addition to a follower makes a tough team. Coupled with my Lightning storm, I can't really be beat (except by dragon priests and probably everyone on the forums)

wall of flames and paralyze...i like to paralyze someone then roast them alive as my evil wizard laughs mwa haha

Fury and the raise spells. Intelligent use of these with stealth, perhaps traps too, and you can kill everything without anything knowing you're even present. Doing so through a whole dungeon with zero detection is kind of a decent gauge mechanism for a shadow mage being good at what they do. My only problem in practice is that due to how long it takes without overt powerleveling of illusion to get the master of the mind perk, this winds up raising conjuration unduly high relative to illusion early on in undead rich dungeons and dwemer ruins. Conjuration is kind of supposed to be subsidiary with illusion being the true main skill (along with sneak) for the type of character I play, but this is mainly a role play nitpick. I can live with stealth necromancer illusionist in practice.

I use mage light to sping ambushes and find bad guys when I don't have Detect LIfe/Dead. I'll fire it down a (preferably) long hallway and watch as bad guys will agro and go after it. They will go to where the magelight stops. I can use that to snipe them, blast them, set up a rune, etc. I also use it to light up an area instead of candlelight. Since candlelight is on you, the bad guys will see you. With mage light I can run off into the shadows if needed.

Fireball and Candlelight.

Fireball because you can take out most enemies with one or two hits directly on them, and also deal damage to enemies around the one you hit.
Candlelight because I hate not being able to see in dungeons, or even just wandering around at night!

Favorite Spell combo- fireball + chain lightning (great for large groups of weaker enemies)

Favorite Shout- Marked for Death, or the Ice one all 3 words, love freezing my opponents and hacking them to death

most commonly used spells- fireball, conjure creature

I havent found every spell yet, and this is my first playthrough focusing on magic more than anything else fully. So Im sure there are a few spells ill end up loving later on.

Dead Thrall and Wall of Storms.