What is the best way to sharpen a knife?

I own a lot of fancy sharpeners, but this is the one that I use the most.

Portable Whetstone

You still need some amount of technique. My own technique is sloppy, so I tried to make up for it by getting expensive sharpeners. Yet this thing gives me the best results and is portable and affordable.

I like to use an orbital sander / belt sander, whatever you have available. Sit the sander on my lap, upside down so that I can squeeze between my legs. Hold the knife with two hands, knife, chisel, hatchet, anything you wish to sharpen, to whatever degree bevel you require, and sand away through the different grits of paper. Start with an 80, move up to 120/180/240/400

On the 80 grit, unless you have a quality knife, and I'd put even cheap kitchen knives in that category. Then you'd be leaving horrendous marks on the blade, in addition to taking a lot of metal off and, possibly, changing the critical temper of the blade from heat build up.

To be fair, a well-used 80 grit belt is not nearly as aggressive as a new one, so might pass as a bastard 120 or 150 grit.

If I went that route, which grit I started with would depend on what I'm sharpening. An ax, hoe or mower blade might play well with an 80 grit belt or disc. For more delicate jobs, I'd start with 320 or 220. I run the latter and a 1"x42" Delta belt sander, and it makes touching up lathe knives a quick task.  In any event, I would move fast to keep the metal from heating.

One of the tips I found from watching youtube videos on this topic is the importance of finishing with a leather strop. Used properly, the strop will deliver a true edge. free from burrs overlapping from either side of the blades' edge.

After decades of sharpening knives with a variety of methods and abrasives, I've settled on diamond sharpeners as giving consistently good results quickly and with minimal effort. The results depend primarily on a few things, namely the quality of the blade steel, the condition of the blade to be sharpened, the quality and amount of diamond dust on the sharpener, and the angle of the blade against the sharpener. It's been years since I last used the Japanese waterstone in the knife drawer, although I acheved good results with it, because it takes much longer than using a diamond "stone" (which wasn't even readily available when I got the waterstone). However, waterstones and diamond stones share the virtue of removing a minimal amount of steel in the sharpening, greatly extending the life of the knife. (I can see using a grinder or belt sander in making a knife or restoring a seriously abused one, but I wouldn't do that for a commercial kitchen knife in good condition.)

I've learned that a key technique for quickly getting a usefully sharp edge (I don't intend to ever shave with a kitchen knife) is ensuring the knife is angled carefully against the stone and that the blade is pushed diagonally onto it rather than drug away. Because I'm old, I need strong light to see the edge well enough to know when it's at the correct angle, just touching the stone. (Increasing the angle is likely to actually dull the edge.) But another way to know the angle is right is by feel. When the angle is too low, the blade just slides. But when it's raised to the right angle, with the edge grazing the stone, I can feel the light cutting action. A few strokes is then enough to restore a "kitchen keen" edge.

I just wonder how this is sharpened.

https://www.youtube.com/shorts/LSJID03a6MI

I just went to cut off the plastic wrap from an all weather electrical cover I was going to install outside my garage.  My knife poked a hole in it but would not cut the plastic wrap on the first 2 inches of blade.
Time to sharpen.
I have an old whetstone that was my grandpa's sitting by my utility sink in the garage.  I gets plenty of use from me.  I am never without a pocket knife on me.  (When we fly somewhere on vacation I feel naked without one on me, but one always goes into checked luggage).
One minute or so and my knife is now very sharp again.
Several years ago a guy I worked with had gone to a garage sale and picked up a bucket full of whetstones for a couple of dollars.  He asked if I wanted any, of course my answer was "YES".  I now have whet stones in all my various tool boxes, vehicles, hunting packs, etc.
Favorite pasttime for me is sitting around the fire at hunting camp sharpening all the knives.