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Diana is scary strong now because the global gold changes means she can have an AP Carry build while jungling. Her shield provides top tier sustain in her first clear, and once she hits level 6, her ganks are amazing, and her damage is just broken for how tanky she is.
Anyways, back on topic, Amumu is still a top tier jungler, and is still one of the best late game junglers with his ult. With more gold, Amumu will be able to be even tankier (is that even possible? haha) or be able to build a little AP to do some respectable damage while still being insanely tanky.
Also, if you want a kind of "sleeper" jungler, check out Darius. He clears fairly well, and can counter jungle extremely well. If you land a pull in your ganks, that'll usually end up being a kill.