1)"Pirate King" Gol D. Roger
The Pirate King. Became PK even with a death threatening illness. Has been referenced many times as being the strongest pirate ever, his strength acknowledged by prominent figures in the One Piece world. I put him on #1 cause DUH!
2)"Whitebeard" Edward Newgate
Dubbed the "strongest man alive" after Roger's death. WB has been the only one to match Roger in strength, getting a draw with the PK. Held the title of Younkou for decades and is referenced as having the power to "destroy the world". Even in old age WB has been able to fight against WG most prominent figures like the Admirals. His DF is just ridiculously strong and has full mastery of Haki which he can use in conjunction with his massive physical strength. Suffice it to say in his prime he was VERY strong. I put him in #2 cause he fought evenly with Roger and his strength in his old days is a testament to his level of strength at his prime.
3)"Golden Lion" Shiki
One of the prominent figure's in Roger's era. Shiki was almost able to fight on par with Roger, and almost defeat Garp + Sengoku. He also was the first person to escape from Impel Down, making his way from level 6. He also amassed the biggest pirate fleet of Roger's era. I put him on # 3 cause he fought the duo of Garp + Sengoku in their prime and almost defeated them.
4)Monkey D. Dragon
The most wanted man alive and father to the future PK. Dragon has been referenced as being as dangerous as Roger, and is the leader of the Revolutionary Army with subordinated as strong as Emporio. Dragon has use of Haki, as seen when he was able to grab Smoker and keep him off Luffy. I put him on #4 cause he's the most wanted man alive with a bounty currently surpassing all other characters, and is implied as being one on the strongest characters. However this one's a baseless statement cause we haven't really seen the extent of his abilities.
5)"Red Haired" Shanks
One of the Yonkou, Shanks is revered as one of the four emperors of the most dangerous ocean in the world. With enough strength to clash with WB, Shanks is feared by almost all pirates. Being an ex-apprentice in Roger's crew Shanks has full mastery of Haki and possesses King's Disposition Haki. Even WB revered Shanks for his Haki when he used it on his "visit". With power to clash with Mihawk, the "strongest swordsman in the world", Shanks has been one of the most prominent pirates for decades. Shank's power is so great that Sengoku acknowledged that if WB and him joined it would be the end of the marines. I put him on #5 for his implied power as one of the Younkou and his King's Disposition.
The second highest command of the WG, Sengoku commands all marines. Sengoku is feared through the world and was revered even by WB as a master "strategist". Sengoku wields a higher position than the strongest fighters of the WG, the Admirals. With enough power to subdue a pissed-off Garp without using his DF when a sad twist of events occurred. Sengoku's DF is very strong enough to take down the execution platform after being blocked by Luffy's Fussen. Being a prominent figure in Roger's era, Sengoku continues to be one of the most feared and prominent figures of the One Piece world. In his prime Sengoku along with Garp took down one of the strongest pirates of the era, Shiki. I put him on top of Garp cause not only did he subdue Garp when Garp was enraged by Akainu's actions but Sengoku had warned Garp that if he tried anything funny at Marineford he would "show no mercy", implying that he could defeat Garp, the legendary marine.
7)"Garp the Fist" Monkey D. Garp
One of the prominent figures of Roger's era, Garp is revered as the "legendary marine". Being the only marine who has cornered Roger many times, Garp's strength was acknowledged by Roger while he was still alive and also acknowledged by WB at the battle of Marineford. Garp also has fought to the death with Roger many times, almost matching Roger in strength. Garp has inhuman strength and inhuman speed, as shown when he visited Luffy at Water 7. Garp has mastery of Haki, as shown when he inflicted physical pain at Luffy and punched Marco while in Marco was in his phoenix form. Garp has also rejected the position of Admiral many times in the past and still remain one of the strongest of the marines. He's also the father of the world's most wanted criminal and grandfather to the future PK.
8)"Dark King" Silvers Rayleigh
The first mate and right hand man of the Pirate King, Rayleigh is also considered to be a "legend". Strong enough to fight evenly with Kizaru, one of the WG 3 greatest powers, in his old days. Rayleigh has mastery of Haki and possess King Disposition, commanding it with such ability that he could target different people and lay them unconscious, not to mention making Kizaru tangible and drawing blood from his face. Kizaru commented that a capturing a man of Rayleigh's caliber would be too "troublesome", while Garp ordered the marines not to engage Rayleigh since he was just too strong. He was referenced by Shakuyaku as saying that he's "100 times stronger" than the supernovas, this of course is an exaggeration but it gives you a hint of Rayleigh's strength. In his prime he wielded enough power to defeat any of the current admirals with his phenomenal Haki. I put him below Garp cause Garp fought par on par with Roger which is obviously stronger than Rayleigh since he's the captain. Also Garp commented to the marines to not fight Rayleigh and that he would take care of him eventually, implying he could defeat Rayleigh.
9)"Blackbeard" Marshall D. Teach
Strong enough to scar Shanks 10 years ago, Blackbeard has been shown to be one of the strongest pirates in the sea. With enough strength to defeat two commanders of WB crew, Thatch and Ace, Blackbeard has been shown to have inhuman strength. Becoming a Shichibukai with relative ease, Blackbeard was revered after defeating Ace, a logia user. His Darkness DF allows him to wield extraordinary power, as he can nullify and drain ANY other DF. With the combination of his DF, his strength/endurance and his experience makes him a formidable opponent.
With enough power to sink an entire island, Enel has a Logia Df with power boasting invincibility. Enel commands enough power to deliver a lightning shock to decimate a normal human being and use his lightning DF to move "lightning fast". He also commands mastery of Mantra which gives him a huge advantage over enemies as he's able to predict fully his enemies moves and read his enemies mind. While it is unknown if Enel has Haki, the Gorgon sisters displayed the ability to predict Luffy's move with the use of Haki, which could mean some relation between the two. In fact, the only reason Luffy defeated him is because Rubber is Lightning's elemental weakness. Oda himself stated that if Enel was a pirate his bounty would be around 500,000,000 which exceeds any of the past Shichibukai's bounties.