Zira seemed to be trying to fulfill Scar's wishes in trying to take over the Pridelands rather than just because of a lust for power like he did. As others have said, she did at times show that she cared even a little for her cubs (especially Nuka), and when she didn't, it was only because she blinded by her loyalty to Scar. Scar, on the other hand, didn't care about anybody, not even the hyenas. He was only using them as his personal army in order to take over the kingdom. He felt no remorse in killing his own brother, banishing his young nephew and trying to get him killed, or even letting the entire Pridelands suffer due to lack of food (remember when he slapped Sarabi?) At least Zira felt sad when her son died.
So, long story short, I vote Scar most evil.

As Taka said, Zira was insane and scary, but Scar was just more evil.