Righty heres my rankings:
2. Big Mam
And I wont rank Blackbeard and will state why.
Shanks - Right before he was claimed to be on par with Hawk Eyes the greatest swordsmen in the series. Before the war he fought against Kaidou and beat him and from the looks of it didnt get a scratch. Even after losing his arm he was able to block Whitebeard attack, cutting the sky in half pretty much which shows after losing one arm he is still a badass.
Big Mam - It's hard to say given that we still dont know alot about Big Mam powers really. All we know shes got the title of Yonkou, which comes with big reputation, lots of turf and alot of pirates under there command. Until I see more of her power in the story I will just stick with this.
Kaidou - I'm claiming he is the weakest at the moment due not seeing alot about him still. As I said above before the war he was beat by Shanks which has to come to something I meantion I remember Garp mention that each of the Yonkou's were in a power struggle were here Shanks showed he can beat Kaidou. Out of the four Law who is one of the strongest Supernovas claims hes there best chance of attacking so he has to have some idea about him too.
As Dag mention I wouldnt put Blackbeard in the power ranking just yet until we see some more of him after the timeskip.
Blackbeard - When we look back to his fight with Whitebeard he didnt exactly show an advantage of being on the offensive did he ? WB totally owned him, yeah his earthquake powers were nullifed due to BB DF but that was it, and I'll quote here, he was a 'damage sponge'. It was also showing during Ace fight against BB he just took loads of damage too with the odd DF attack. BB raise to power is through fear of reputation during the timeskip. People would have heard of his fight at the War and obviously thought he was the next WB or Roger so to speak. Because of his knowledge of Wb turf he was able to move quickly and claim the 'best spots' and armed with the inmates from Impel Down who all must be crazy strong only increases the fear hes spreading around.
But yeah there my opinions.
: My bro is dead. He's gone! But he's there on my back, and here in my heart! He lives on as a part of me! If you're gonna dig, dig to the heavens! No matter what's in my way I won't stop!! Once I've dug through...it means that I've won! Just who the hell do you think I am? I'm Simon. I'm NOT my bro! I'M ME!! SIMON THE DIGGER!
: Your drill is the drill that will pierce the heavens!
(All quotes from one of my favourite Anime Tengen Toppa Gurren Lagann)