The implication of Cain is that when he was younger, he was the best. He was the League of Assassin's first stringer (back when Bronze Tiger and Shiva were up-and-coming rookies, not yet as great as they'd become), he was one of whom Bruce Wayne sought out for HtH training, trained Deadshot, Ra's gave him carte blanche for a project to make the ultimate assassin twice, etc..
Now he's a bit slower and all, as he's well into his 40s, but still pretty formidable and probably still in the top five assassins on the planet and someone that A-listers take very seriously even now.