Dr. Strange has some incredible highs, it's true, but he's also got some of the lowest lows around. Getting beaten by Spiderman, or knifed by a random dude, getting beaten by Thor, etc.
And some of the feats that have been mentioned thus far -- Fighting Adam Warlock with the Infinity Gauntlet, for instance -- are accomplished with prep, and by bringing in every mystical artifact he has, rather than just the 'standard' artifacts he generally carries. If you're going to go in THAT direction, you may as well point to Dr. Fate destroying an entire UNIVERSE in Cosmic Odyssey. Some of them are grossly mis-represented, like 'fighting' the Living Tribunal (yes, he attacked him -- so could Paste Pot Pete).
In general, it seems pretty clear, especially from the JLA/Avengers crossover, that Dr. Fate is going to have more raw power immediately available to him. He himself is an incarnate Lord of Order (Or balance, depending on the version we're talking about), where as Dr. Strange generally has to draw upon the powers of others to effect his spells -- thusly is more reliant on time, on words, on gestures. Dr. Fate also has the added advantage of being physically superior -- invulnerable and super strong -- which in a battle like this may SEEM like a small thing, until you realize the importance of striking first, of being able to take more punishment even as you dish it out.
That said, I don't think anyone can doubt that classic Dr. Strange has some of the highest feats in comics, and his highest highs are higher than those of Dr. Fate, IMHO.
It's probably a wash -- both I see as above top tier and the respective pinnacles of their universe's magic.