Planning on enchanting a robe, I want to know what spell is best, thanks.

for defense any of the elemental shields are good. you could also just boost your magicka.

Both are good.

if you want to go for the power of your spells, I'd say enchant the magic skill you use most spells off.
increasing that stat, should bring the cost of spells in that section town. (it does not increase the power of the spell)
That option is more powerfull, but will limit you to spells of one group.

If you rather want to increase your manapool (the other way around) enchanting your gear with increase magica can be a good idea too.
(increasing intelligence is less effective to that end)

Finally if you don't want the big spells but already have a sufficient manapool to do your thing, (but drain it empty to fast) you may beter go for willpower or regenerate mana (it depends on your current manapool what is best of those 2 for that)
(regeneration is also increased by the size of your total manapool)

Spell absorbtion can be smart to boost if you cast a lot of boost yourself spells. it is possible to get 101% spell absorbtion, making casting buffs on yourself free:)

so it depends on your magic style what you should pick.. personally I'd not pick any of the shields... that will only help your defence... but you want to do damage don't you?
and if you MUST do defence.. it is possible to get to almost 100% magic resistance AND 100% normal resisstance, making you near or complete immune to all attacks.
(but personally I find this makes the game dull)

want that same effect but with a little challenge? 100% charmeleon can be aquired...

Elemental sheild or Fortify Magicka. This has already been stated, but what wasn't was Sigil Stones. You really, really, really want those enchantments coming from Transcendant Sigil Stones that are found at levels 17+. Grand soul enchantments just don't come close in this case. So hit level 17 and start closing Oblivion gates for a stash of these things.

For Robes you really have to love the look to use them. You lose an enchantment slot by using robes when compared outfits that have separate pieces for torso and legs.


Minor note: There is two 25% elemental shield Sigil Stones (Shock & Frost) that can be found in the level 9-12 (Latent) range.

Elemental Shields are great, since they'll offer some protection from both magic and physical attacks. And, since they're not more expensive to enchant (especially if you're using Sigil Stones), there's really no reason to ever use a plain Shield enchantment. In fact, in a lot of ways the elemental shields are better than wearing regular armor:
1) Clothing doesn't weigh as much (and some of it is totally weightless).
2) Clothing (and their enchantments) don't get damaged or broken, so you'll never lose any defense rating in combat. Traditional armor often gets damaged really quickly, and you can suddenly find yourself taking huge hits mid-fight (not to mention the hassle of having to carry lots of repair hammers to repair frequently, though that does help increase the skill faster, and thus bump up your Endurance).
3) Clothing doesn't slow down your running speed, or negatively impact your ability to sneak.

If you don't want to use the Elemental Shield enchants (or if you've already reached the cap of 85% damage reduction), some other worthwhile ones to use are +Magicka, Spell Absorption, Reflect Spell, and Feather (which isn't particularly helpful in combat, but is nice for convenience, especially if your Alteration skill is low enough that you're unable to cast good Feather spells).

Enchant a robe,hood and the wrist irons with the three Elemental Shields.Not only will it add to your elemental resistance,it will add to your armor rating.Also add a physical shield enchantment to your shoes.

Like some of the others have mentioned, I'd go with a Transcendent Sigil Stone. You can get some really great enchantments from these. Here's my list:

1. Offensive - Fortify Magicka 50pts

2. Defensive - Fire/Frost/Shock Shield for 25pts or Resist Magic for 20pts (use if your character is NOT a Breton with the Mundane Ring) or Spell Absorption for 15pts.

Cheers

Yes, a Transcendent Sigil Stone, for Willpower.
Use a few of them, 4 or 5, on common rings and amulet too.
Get your willpower up to maximum, then watch your Magika refill nearly as fast as you can use it!