Ranks are all relative though. Some characters just have more advantages over others. Another way to look (though I'm a bit surprised that Unohana is barely below Yamamoto and Aizen considering everyone considered her "weak") is just through relativity relationship in strengths and weaknesses.
-Yamamoto: Perfect in everything except physical strength (he's an old man, he can be forgiven)
-Soi Fon: Strength and motion are weaknesses but lacking in defense, kidou and smarts. (makes sense: she relies mostly on flash steps and martial arts motion)
-Gin: Well rounded in everything, yet not perfect in everything. (as weak as everyone makes that to be, that's rather dangerous considering he's not weak in any particular thing)
-Unohana: A bit surprising: perfect in attack, kidou and knoweldge, nearly perfect in physical ability (she never showed any I know), with movement being her only real weakness.
-Aizen: Similarly surprising: as perfect as he claims he is, his power is mostly in attack, kidou and knowledge (like Unohana), but even his weaknesses is still pretty high. If Yamamoto was a bit younger he probably would beat Aizen though :p
-Byakuya: Not really perfect in anything, but close to it in attack, movement, kidou and knowledge with physical and defense being his only problems.
-Komamura: A beast (literally) in attack, physical abilities and defense, yet kidou and movement really suck.
-Shuhei: Sort of surprised that his perfect ability is kidou, with nearly close in everything except physical ability. (that's a pattern for many)
-Tousen: Close to perfect (but not quite) in both kidou and knowledge, yet physical, attacks and defense not up there. (and it shows…but as we know this probably changed after moving to Hueco Mundo)
-Hitsugaya: Reminds me of Gin's, only better than average movement and kidou abilities. (I still say the shrimp cheated though)
-Kenpatchi: We all know his story: perfect strength, perfect attack, good defense...yet no kidou whatsoever and horrible with knowlege. But Kenny is mostly for hacking and slashing so who cares.
-Mayuri: Similarly we know him: brilliant on knowledge and even kidou (surprised again), yet crappy with movement and just as bad with strength. But with his weird abilities, who needs that.
-Ukitake: Interesting what the sick guy has in him: perfect in knowledge and kidou, nearly with attack and defense, and strength, as always, his weak point.
Personally wish they had the other Shinigami in there like VC ranks to see how close they are to the captains and their own potential...but hey, at least we got this.