I don't care if the Alpha Legion is portrayed as being superior. Whether we like it or not, the Legiones Astartes are not all equal. That has become more and more obvious with the release of the Horus Heresy novels and the Forge World books. Think about it objectively:
1. The Iron Warriors are portrayed as specialists whose relationship with their increasingly volatile primarch is complicated at best.
2. Once reunited with the Night Haunter, the Night Lords swiftly became a legion of murderers and rapists who generally didn't trust one another.
3. The Flaw forced the Blood Angels to become specialists as well. It also ensured that their tactics and strategies could never be flawless: a certain percentage of their force was increasingly susceptible to losing control.
4. The Iron Hands were a very capable legion, but they were ultimately an overt and blunt instrument geared toward overt and blunt modes of war.
5. The World Eaters were lobotomized berserkers.
6. The Death Guard were specialists in perhaps the most counter-productive mode of war in the 31st millennium.
7. The Word Bearers were numerous, well-equipped, and extremely motivated, and nothing indicates that they were sub-standard in any mode of war. Ultimately, though, they don't strive for the same level of synergy and cohesion that has become the hallmark of the Alpha Legion.
So, right off the bat, that's seven legions that possess flaws that put them a peg below the Alpha Legion, or lack the merits of the XX, or simply pursue a mode of war that ensures they'll take more losses than they'll dish out. The serious debate, in my humble opinion, only starts when we start comparing the Alpha Legion to the other remaining ten legions. Even then, though, I think we can objectively see how even some of the great legions with great primarchs simply don't strive for the synergy that is the Alpha Legion's hallmark, and all but one approach covert warfare as an afterthought - if at all!
My short list for legions that could go toe-to-toe with the Alpha Legion are the Dark Angels (because it's implied that they are one of the few legions that doesn't specialize, and are capable of performing any mission set), the Thousand Sons (because they are game-changers in their own way), and the Ultramarines (maybe; because they, too, have excellent synergy and cooperation). I would add the Sons of Horus to that list, but I honestly think they would only best or match the XX where Horus was. I don't think the average company of the XVI would be prepared to deal with the twists within twists within twists that the Alpha Legion do so well.