What are the best enchanted items in the game?

Tough to say, so I'll answer the only way I know how - with wiki links.

Some enchanted items are not random and can http://uesp.net/wiki/Oblivion:Enchanted_items_in_fixed_locations.

Otherwise, you can get given items as a rewards for completing missions, most often from the http://uesp.net/wiki/Oblivion:Artifacts

If not, go make your own weapons and items http://uesp.net/wiki/Oblivion:Altar_of_Enchantment#Enchanting_using_Altars Get some ideas http://uesp.net/wiki/Oblivion:Useful_Enchantments

Some are levelled, some are not. It's tough - you can't really say what's the best, because it matters differently to some. Do you want a necklace that protects you, or a sword to fight with? Sorry I can't answer you more directly, but it's really quite a tough one and I don't want to suggest one particular thing.

A matter of opinion, I am afraid, you can only really say what is your favourite, and also depends on your character class, for example I love the Birthright of Astalon, but obviously not with a heavy armour character.
Personal opinion, sad but true I go for what looks cool to me, so Birthright, self-enchanted Elven shortswords, the Ebony Blade and Spellbreaker.

Very much depends on the nature of the character involved and personal opinion. That said, Azura's Star is the only enchanted item my character cares about aside from the few she enchants herself. I should think that most any character who uses a magic weapon would find the Star invaluable.