I think that both Moria and Croc are underestimated by a lot of people.
I think a lot of people underestimate Croc because in the third fight, he got annoyed, said he'd fight Luffy "as a pirate," and stopped using his powers, until near the very end. It wasn't that he was limited in his moves because of the tomb, he purposely decided not to use them. At least that was the impression I got. Maybe I'm wrong.
But assuming I'm remembering things correctly, it means something interesting. It means Croc never lost a single fight in the entire manga when he was going all out the whole time. That kind of makes it tough to really gauge his powers, doesn't it?
Now, I know that Luffy had nullified Croc's defensive ability anyways to an extent, but how much of a difference would there have been had Croc used offensive abilities of his fruit the whole time? Think of the Ace/BB fight. Now, I know Ace lost anyways, but imagine how badly he would have done if, after seeing BB hit him, he had simply said "Well, since you can hit me, I think I just won't use my fruit at all. Have fun with your fruit though, use it all you want."
As for Moria, his shadow power let him create an army of Zombies. Which means, in a sense, every single zombie in Thriller Bark, including ones like Samurai Ryuuma, were a part of Moria's power. Yah, Moria had Hogback's help with the Zombies, but I don't think that should count against him too much. Think of the corpses Moria used as tools made for him by Hogback, similiar to the way Nami uses the Clima-Tact made by Ussop, or the way Zoro uses swords made by (insert whatever famous swordsmiths made them.)
Thus, I think it's perfectly fair to consider all the Zombies simply part of Moria, so if you want to pit Moria in a one-on-one fight against another Shichi, you'd have to give Moria his entire Zombie army to be fair. And that suddenly changes things, doesn't it? Even if you ignore Odz and simply give Moria the Zombies he had available at the start of the Thriller Bark arc, it's still a very powerful army. The weak ones are just fodder, but the General Zombies are quite potent, and all of them together with Moria himself would be tough to fight.
And the interesting thing is, every time Moria defeats a strong opponent and steals their shadow, Moria increases in strength. Right now, he's pretty weak though, since he lost his whole army.