This ^ ! If we're talking "perfect" there cannot be only one. The perfect one will have the length, the weight, the type (fixed or folder, one handed or two handed opening, etc.) that match exactly my requirements of the day. And it must please my sense of aesthetics. Important for an EDC. This said, I am very at ease (currently !) with 2.5" to 3" or 3.5" blades, fixed or folder. My uses have changed and I don't reach so often for my 4" and + blades anymore. If we're talking "most used", that would be around 3". But of course, my EDC is an assortment of 2 to 3 knives, ordinarily. Let's say, these, as an example for fixed (in the right pants pocket), in (from left to right) 2.36, 2.64, 2.76, 3.35, 3.43 and 3.74" :
And these, for folders (in the left vest pocket), as an example, in (from left to right) 2.56, 2.64, 2.68, 2.76, 2.91 and 3.94" :
But there is one I actually carry everyday as a necker, this one with a 3.19" blade. Probably wins in the "most carried" category :
And this one is always there, in the purse, as a backup. The small model (below), blade is 2.91" :