I won't mention Priest , Maiden, Sabbath because they have a class all their own, and there needs to be no further discussion on how much they have done for metal.
Motorhead-I am not sure exactly when they got going, but if they fall in the 25 year mark..they should be here. Lemmy and his croak, dirty, gut level metal..that simple.
Venom/Bathory-these two bands pretty much got the whole Black metal thing going. Loads of Norweigans and other Scandinavian bands owe these guys.
Slayer-the fastest and most crushing of the Thrash scene. HIGHLY important to modern death and black metal.
Death-Pretty much started Death metal. RIP Chuck
Emperor-Pretty much THE Atmospheric Black Metal band. Plus, they never stagnated..the band evolved over 4 albums, continuing to push the limits.
(I am not a expert on Black Metal..if other bands did that sound first and better..correct me)
At the Gates-The band that kicked off Melodic Death. Often copied, but never replicated.
That's just off the top of my head