Not too big, not too heavy, easy to handle quickly, and not a folder. The rest is preference IMO. Somebody with way more knowledge and wicked knife fighting experience will be along to give you theirs.

I only carry one knife; a CQC from Emerson and I've been very happy with it:

http://www.emersonknives.com/ekTheKnives_Tact.php

The waiting list is rediculous, but they make great knives.