Board Thread:Questions and Answers/@comment-6673698-20130324101714/@comment-7658432-20130410071957

What a question... It's like asking "Is Ganju stronger than Aizen?". At least ask a question with a good basis. Well, since you've asked for it, I'll give you a detailed list.

Speed - Ulquiorra

Physical Strength - Zaraki

Reiryoku - Zaraki

Endurance - Ulquiorra (via High Speed Regeneration)

Intellect - Ulquiorra

Zanjutsu - Zaraki (I just feel that Ken-chan would win in a fair swordfight)

Powers (Cero, etc. I don't know how I would say it)- Ulquiorra (since Ken-chan doesn't use Kido)

See? Biased answer. Who's the stronger of the two, I leave that to you.