I personally used...
- Apprentice: Skeleton obviously to start with, thought it was quickly replaced with the Zombie for foes that did damage without weapons like wolves and such and I used the ghost against weapon wielding foes since it couldn't be harmed by most low level weapons, then came the scamp for my ranged needs. So 3 or 4 depending on if you look at the skeleton as being useful or not, it's obviously the best starter though when you have low mana due to it having the lowest manacost for the most amount of time (40 seconds base).
I suppose you can justify using the ghost, since it's pretty similar to the scamp and has normal weapons resist. However, it's super slow to move around, making it do less damage AND making it a limited meatshield. But yeah, I concede that maybe that one is useful. The Apprentice level spells were most balanced, but you also stay with them the least amount of time.
- Journeyman: Pretty much everything here like with apprentice, I still threw the ghost at some enemies with weapons if I only needed it as a distraction but the Dremora came when I had a lot of mana at the ready and needed a powerhouse, the Headless Zombie replaced the normal one, flame atronach for scamp and then came my favorite skeleton the Skeleton Guardian that came with a leveled shield! awesome against ogres and other hard hitters. Definitely all 4 here.
Headless Zombie just IS the ONLY choice here. The Skeleton Guardian does 8 damage a hit. The Headless Zombie does 27. It also has 5 more hp and some magic resist. There is just no competition unless you don't have the mana to cast the Headless Zombie.
Flame Atronach is pretty hard to justify using at this point as well. It does 5 points of melee damage, and has spells that do either 8 on target or 13 on touch. The fact that it's ranged is nice, but again, the headless zombie does 27 damage! It'd be silly to use the flame atronach in almost any case. Similarly, the Dremora does 5 damage in melee and has a couple random apprentice level spells. It also costs more than the Headless Zombie. I really don't see how it could POSSIBLY be construed as worth using.
- Expert: Here came my personal favorite of all of the summons, the Clannfear, Faded Wraith came instead of the Ghost, Frost atronach if I was against something that didn't like frost or wielded frost itself and sometimes just as a shield (huge and can heal itself...), I wasn't too fond of the Daedroth and Spider Daedra as they were always so slow to get to the fighting and I felt the Skeleton Champion cost too much considering I could make a Dremora and the Skeleton Hero was quite pathetic, no shield made it worth less than the skeleton guardian for me which I used instead. So I used personally 3 at this level.
This is, of course, not particularly different from what I said, except that I actually use the Daedroth. Not much to be said about this.
- Master: Dremora Lord... need I say more ? Then an upgrade to the wraith with a Gloom Wraith, Storm Atronach to have the shock element covered and Liches are always awesome, only one here I never liked much was the Xivilai, just used Dremora Lords instead. So 4 here.
The Gloom Wraith is a really annoying summon though. It CAN do a ton of damage, but all too frequently, it flies around super slowly trying to get into melee range. It's too slow to get near stuff that much, and when it DOES, it only does 17 damage, much less than the 35 of the Clannfear or 40 of the Daedroth. Sure, it has very powerful spells, but it honestly doesn't frequently use them enough to get anywhere near the damage output of the Clannfear and Daedroth. Meanwhile, it costs a TON. I dont find that the Faded Wraith does much worse in terms of damage because while its spells are not as good, it actually does more melee damage.
The Dremora Lord can be good. There are a lot of variables as to the spells and stuff it comes out with, but at its best, it still only will basically match the Clannfear and Daedroth in damage while costing a ton more mana.
The Lich is also hit or miss but mostly miss. It gets a random staff, but frequently the staff is virtually worthless. When it IS a good one, it can be powerful, but even then, its insanely expensive and can frequently end up flying around slowly doing nothing the entire time. It's not worth it.
The Storm Atronach is not BAD, but it is not good enough to justify using really. It does 26 damage in melee instead of the 40 a Frost Atronach does. Meanwhile, it's spells are of similar quality. Unless you are fighting a frost resistant or shock weak enemy, the Frost Atronach will be better. Meanwhile, that is not even mentioning that the Storm Atronach's most powerful attack does 30 damage, while a Clannfear and Daedroth will always be doing more than that, and while the Storm Atronach can do ranged attacks, the Clannfear and Daedroth will probably still get off more attacks because they are faster.