To me, the all time best single piece of armor is the Haunted Helmet. Nothing before or since has had the "cool" factor of it. Combine it with the Eternal armor effect and you have a blue burning skull inside a helmet. If that isn't cool, I don't know what is. Really, I think Reach was the peak of armor design in the series. You of course have classics such as the MK V and MK VI, but also a whole slew of great armor. Obviously what looks good and what doesn't is subjective, but all the armor is Reach to me looks like a Spartan would actually wear it. It looks like the armor is meant to serve a purpose and the looks just happen to be a result of that.
A close second is the OG Recon set from Halo 3. I still remember getting Recon with my friends as a kid and the genuine excitement of equipping it for the first time. Again, as far a cool factor, Recon set with Katana is pretty darn cool. Aside from looking good, it also meant something. Having full Recon set and the Katana meant that you had EVERY achievement.
Broadly, I think armor stopped looking good after Reach. I do not like 343's style and I honestly think all of their armor is over-designed garbage.