Well, the magic in Skyrim Vanille is kinda smoother than Oblivion Vanille. Playing a mage character in Oblivion can be a death sentence, since Oblivion's magic is built more as a tool of pure support, whereas Skyrim's magic can be (or at least look) real independent from swordplay and skulduggery. But Oblivion's magic is definitely more colorful than Skyrim's, especially with the addition of my favorite part in Oblivion's whole system, them skills/attributes. That said, I do prefer playing both magic systems modded: I use Supreme Magicka with Oblivion to power it up, and I use Requiem with Skyrim to make spells feel less like an action game and more like an actual RPG.
Morrowind's magic, though, with the names and the crazy levitate and Mark and Recall (with no Fast Travel!) and that spell that made you jump from Seyda Neen to Vivec City, 's definitely the best.
Anyways, like I said, I decided to play Oblivion instead of Skyrim for this playthrough. Relatively unrelated comparison, as I'm goin' along, I'm noting the generally more immersive quests of Oblivion: Quests definitely don't feel like they all end up hack-and-slash, like Skyrim (eg. Thieves' Guild: In Oblivion, outside of the special quests, to really advance in the guild you just have to steal poop; in Skyrim, all you have to do is special quests, with only a terrible few of them being pure and bloodless theft missions. Ie, Oblivion's TG actually feels like it's stealing shit, while Skyrim's TG is more like a Fighter's Guild which likes using bows and stealth). And definitely, though a lot of the dialogue in Oblivion feels rather inconsistent compared to Skyrim (especially the damn rumor system and the NPC2NPC interactions), I kinda feel less compelled to just skip through what they're sayin' in Oblivion than in Skyrim, by virtue of the words being more entertaining (or at least more fulfilling, for there are a lot of quests in Skyrim where actually listening to Quest NPCs is unnecessary). Skyrim's quests, however, have the noteworthy virtue of actually feeling urgent, whereas a lot of the quests in Oblivion (and, rather tangentially, most quests in Morrowind) you can delay for long periods of time even right before their climaxes without feeling bothered at all.