People sure are giving some bizarre answers to this thread - Metallica? Megadeth? Slayer? Bathory? Morbid Angel? These are/were all fantastic bands, but none of them have released anything 'influential' in the past 15 years. 15 years ago was what, 94/95? How could anyone say anything Metallica have done since then has been anything approaching influential? Also, it seems a lot of you don't understand the difference between 'influential' and 'good'. Porcupine Tree are a fantastic band, but it wouldn't be true to say they've been influential. Who, aside from Opeth, has claimed PT as an influence? For what it's worth, i'd say the most influential metal bands (on the understanding that 'influential' is distinct from 'good') from the past 15 years were, in no particular order:
Pantera - I don't even like them much, but there's no denying they had a huge influence on metal throughout the 90s. So many modern metal bands owe them a huge debt, particularly with regards to Dimebag's guitar style, which was very distinctive and has been repeatedly copied by innumerable bands.
The Dillinger Escape Plan - There must be about a million DEP copycats doing the rounds these days. They've been hugely influential, and pretty much spawned a genre of metal in themselves.
Korn - Some of you have been saying they shouldn't be anywhere near a list like this, but that's just plain denial. They were massively influential during the second half of the 90s, Christ, it seemed every new band that emerged for a few years back then sounded like a damned carbon copy of Korn. Blech. Influential yes. Good no.
Tool - As much as many people hate the rampant fanboyism that surrounds them, Tool were almost single-handedly responsible for putting prog-rock back into the mainstream. Their influence can also be heard in countless bands since the mid-90s onwards (and even more so after Lateralus was released), from their esoteric lyrics, artwork through to Adam Jones' guitar style and Maynard's vocals.
Meshuggah - So influential they've even influenced Tool (see 'Jambi' from 10000 Days). It seems like everyone's trying to insert a bit of Meshuggah into their sound these days - see the final minutes of Opeth's Deliverance. And then there's the sudden proliferation of bands who sound very similar only not as good.