Looks and/or actions-just the first pair that pops into your head…
Meep from Phineas & Ferb, and the head alien from Mars Attacks
E.T./Roger (from American Dad)
Oh, wait…you said evil, not snarky!
E.T./Xenomorphs from Alien/s/3/etc.
Starman/Galactus
Most evil? Those flying parasites from original series Star Trek that infest people then control them through pain come to mind as a candidate.
Nicest? Those hedonistic Baywatch extras from the TNG episode “Justice”.
Nicest: The pleasure GELF on Red Dwarf
Most Evil: Too many to choose from, but the “To Serve Man” guys on The Twilight Zone were pretty nasty.
You mean the ones from the Planet of the Blond Joggers? 
E.T. for nice; the aliens in Independence Day for nasty. The later were nasty just for the sake of being so.
Mork/Vogons
You decide which is which. 
Nicest? Princess Leia, ROTJ, as Jabba’s slave. Never a nicer alien have I ever seen.
Most evil? The Face Hugger from Alien. As a child it was the fear of face huggers getting me while sleeping that kept me me up at night. Evil Bastards.
The Starmanwas a pretty nice guy.
The Brain from Planet Arouswasn’t very nice at all.
Nicest, Superman
Most Evil, the Thing (from another world)
My uncle Martin. Oops, nicest.