I really like Nirvana's "Lake of Fire". "The Man Who Sold The World" is great too. It seems that Kurt's voice fits those songs
I also like Fiona Apple's "Across The Universe" cover. It doesn't sound better than the original, obviously, however it's a good one.
Mihimaru GT's cover of "I should be so Lucky" sounds...a lot like their own style, which is great, because they've added their "signature" to it. And it's in Japanese.