Em is definitely badass.

Kirk Hammet is a great power chord.

I vote for any maj7 chord. Happy and sad at the same time. Used in proper context it can't be beat!

I'll go for two simple ones. C7maj and E7. When used properly, these can make chords progressions obsolete, making the perfect transition with just one chord.

C and G or G and D are magic. Anything combined with its 4th minored is bluesy funky magic. Let alone the branched chords

C, D, E, F, G, A, B:

C-Gm (Atom Heart Mother)
D-Am
E-Bm
F-Cm
G-Dm (Any Colour You Like)
A-Em (Breathe)
B-Fm

...that's not cool, man. Not cool.

G C F

My favorite chords.

Bm

is a great chord.