Straight from stephens translations:
Chapter 303, page 19
_[Grand Line, Marine Headquarters]
Officers: …Vice Admiral!! He... he's not here!!
I'm not settling for that!!
Where the hell did he go?!
His bicycle is gone, so... probably to sea.
Tell the Gorousei at once!!!
Gorousei: ...him again?
Well, it's probably only reconnaissance.
Honestly, what does he consider his own position to be...?
He can't just act on the spot like that.
The Marine Headquarters' Strongest,
"Admiral" Aokiji…_
And chapter 319, page 04
_Sanji: A…
...Admiral...?!!
How high up is that?!!
Robin: The number of Marine officers who hold the rank of "Admiral"
is only three...!!!
"Akainu."
"Aokiji."
"Kizaru."
(Note: Red-Dog, Blue-Pheasant and Yellow-Monkey, respectively.)
Robin: The only person above them is Sengoku, the head of the Marines.
He's one of the three men... called the "Greatest Powers" of the World Governments!!!_
So much about his battle strenght among WG and marine ranks. B)
As far as him being the second most powerful in the entire World is concerned I just have to say that. The WG has the entire marine including the Three Admirals, the Shichibukai and the CP´s at their stake. And yet if Whitebeard and Shanks would work together they consider it as a serious threat. I don´t know but I interpret it that if those two pirates (and crews) would work together everything what the WG launches at them won´t be enough to stop them. Just a little food for thought. ;)