for greatest sensitivity (to most easily feel that sometimes precious, gentle, and short-lived nibble) what combination, in your ever-so-humble opinions, of action and power will provide the best sensitivity?

To answer this question several factors must be taken into consideration. What conditions you are fishing in, ie fast water or slow water (dictating how much lead you are going to use), what kind and how big are the fish that you are fishing for and do you to prefer to control the fish or have the fish dictate what is going to happen. Slow action light power rods load up faster with less weight and too much can give your rod a dead feel. Fast action heavy power rods will feel like a broom stick unless they are loaded properly with the right amount of lead (which may be way too much to fish properly with in some situations). The most important thing is the right tool for the job at hand. If you have to use so much lead that your rod feels sluggish and has a constant load into the mid section you are using the wrong rod for the situation. For most of my fishing I use a fast tip light to moderate power. I feel this combo fits most of my needs when banking it or drifting on the rivers that I fish. When the water is low it is loaded a little light but a fast tip will let you feel those little nibbles because it will allow for a transfer of small vibrations well. When I am banking it and the water is fast and high my rod is loaded a little heavy but again the fast tip because it is a little stiffer than a moderate or slow tip compensates for the over load by taking some(slightly) of the burden out of the mid section of my rod allowing me to have that quick snap that I like to set the hook. IMHO it is the best all round combination for my kind of fishing. For pulling big kings out of rivers I like a heavy power rod with slow to moderate action. The tip loads with less lead and you have the backbone to pull those fish and turn them when needed. Now with the slower tip you don't get as much vibration transfer but when a king hits it is more like an earthquake than a vibration. LOL I just love it, WHACK, rod almost slaps the water, KING ON!!! I don"t know if this will help you or not or even if anyone agrees with me but that's my story and I'm stickin to it because it seems to work for me.