If we were talking about brilliance, I'd definitely give the nod to Scar, but when it comes down to just being pure evil, I have to go with Zira. ^^
Scar did, in fact, actually succeed in killing his brother, but he really didn't do much of the dirty work. All he did was toss Muffy off the cliff after he had been weakened by the wildebeest stampede. As for the rest of his plot, he mainly just pulled the strings, which is more clever, than evil in my opinion.
Given that Zira didn't actually kill anyone, she still was a little more active in her plot to kill Simba. Yes, she trained her own son to do what was needed, which is really evil if I do say so myself. Zira struck her own kin, which again, if that's not evil, I don't know what else is, and furthermore, she was willing (or so it seemed) to even kill her own daughter to get what she wanted.
Don't get me wrong, both Scar and Zira are as bad as baddies come but I just feel as though Zira has a tad bit more evilness tucked inside of her. Her motives just seem more enduring to result in how evil she was for me. Plus I think it takes a lot more to turn a female that evil than a male. Not trying to sound sexist of course, just saying is all.