Optimally, one should be able to reach one's knife with either hand without the need to hold it with the other hand. It should not only be easy to get to but also easy to put back. It should be out of the way of entanglement hazards. It should also be cheap, i.e., it does not need to be titanium. Also, they should be blunt tip so one does not stab one's self.
I have big hands. I need a knife with a decent sized handle. A medium knife is enough. My main knife is a Wenoka EZ lock ($30). It is attached to my console. This particular knife is easily removed and replaced in its sheath. But it is usually a two-handed operation. I use a UK Remora ($24) as a backup on my waist belt. It is small but shaped well. It is easy to get to but difficult to replace.
I find that attaching a knife to my deflater or BC causes the knife handle to stick out and is almost impossible to put back. Try it. Also, I have found that trying to get at the knife on the inside of the thigh is very difficult with drysuit, et al. If attaching to calf or thigh, one should use surgical tubing to take up for compression.