I've gotten one Mal in ~25 runs with my (IK-Armor)-Concentrator. It's kinda easy, and It doesn't hurt my exp-bar either, even though I'm lvl 80. Plus, all the Champions and Unique Monsters usually give me something too
I have had the countess drop absolutely nothing before as well, but that's due to the bug where if she's close enough to a wall or you or the chest in the middle or ANYTHING restricting her movement, she wont drop a damn thing. It's kinda like shooting one of those Cloud Stalkers in Act 3 when it's above the river; nothing will drop because there's no space to drop.
Concerning the Countess' drops, well, I have found an Ist and Um from her. A Pul as well. I've found dozens of Lem-and-belows. I wouldn't count on anything above a Pul happeneing all that often though. It's probably been 300 runs since I've found a Pul or higher. I have, however, found 2 verdungo's from uniques on my way down to her.
The unique and champ monster packs in the Tower Cellar levels are awesome. There's usually 3 or 4 per level (and theres five levels!), and theyre easy to kill. I have two sets of gear on my countess sorc: one set with skills and faster cast rate for just straight Countess slaughtering, and one set with Tal's 3 piece for MF for killing along the way.