Akainu is the man. I'd put money on Akainu against any marine dead or alive with the sole exception of potentially Kong and Dragon (my head canon he used to be Admiral White Dragon and defected before Kuzan became one). That's including Garp, Sengoku, Tsuru, Kizaru, Aokiji, Fujitora, Green Bull & Tensei Kurouma.
I suppose the WG really should have appointed Kizaru though. Where as Aokiji would be to goody good for their taste, Akainu has proven to be a problem as well for the top brass. He is one to lead, not follow. You can't put a man like that as Fleet Admiral and expect him to do your dirty bidding, he wants to lead his way. Kizaru would in that aspect have been the perfect choice for the WG. Indifferent and loyal cog in the machinery that would have done the job, no questions asked. Sengoku was more of a diplomat and strategist but apart from that Kizaru would almost have been like for like replacement until someone new came along.