i mostly agree with mickdawgx81. I haven't seen Shanks going all out so far, have you? As for Mihawk, i don't think he doesn't take shanks serious. I think they are old rivals /friends (Mihawk going to Shanks letting him know about Luffy - that he even knew about Luffy & Shanks is a hint for a deeper relationship. Or when he left the battlefield after Shanks arrived he stated that he agreed to fight WB, not Shanks).
As for BB - he was able do wound Shanks, before he got his ability. He was able to defeat Ace, without WB ability. Now he has his ability, the earthquake power (which he could use to "destroy the world, according to the "Great buddha") for two years and who knows what other abilities he already collected (Jinbey stated that they are hunting for strong DFs and BB is the only one i know about who could use more than one DF power). I think BB is right now the strongest of the yonkou.
As for Shanks: i think he is one of the strongest, but i agree with mickdawgx81 that Roger would be the one to fight WB, not Shanks. Shanks wanted to stop WB back when they met. They clashed swords with each other and i don't think that Shanks won. Maybe he had a draw. A draw against an aging WB, who was the only real rival of Roger, the pirate king (as i know so far). So i just don't see Shanks on a levl with Roger or WB back in their "prime time".
As for Shanks stopping aikanu: When shanks arrived, aikanu already fought Wb, Jimbey, Marco,......and he was just stopped, not defeated. And aikanu was stopped by marco and that swordman of WB crew, too. I doubt BB fled because he thought he had no chances of winning - the loss would have been too great (going thre w all the trouble collecting his crew and maybe loosing them fighting someone that strong - goes for both Shanks & crew & aikanu)
