For keeping a knife sharp, the Spyderco Sharpmaker. But if you let your knives get really dull or you want to reprofile the edge, or you need to sharpen out some damage, the Sharpmaker really isn't fast enough and you will drive yourself crazy.
In addition to the Sharpmaker I recommend a good strop or 3 and diamond spray in several sizes though just some 1 micron is plenty to start or if you're not sharpening crazy yet. For today's high carbide content steels, especially the ones considered 'super steels', it makes a huge difference. Some people do just fine with the other various colored compounds and rouges, but I didn't have great luck with them and often rounded the edge and made my sharp knife dull. The diamond spray was a night and day difference for me and one of the best purchases I've made to go with the hobby.
I've got the Sharpmaker, Edgepro Apex, a guided clamp/rod/slot type (brand?), and paper wheels mounted to a bench grinder. I haven't used the others in a long time now that I am in the habit of touching up knives after every couple days of actual use. With the high wear resistant steels that take a long time to dull, I can usually just strip on the 1 micron diamond and be back to tree topping sharp after a few minutes. If not a minute or so on the fine white Sharpmaker rods is usually enough unless I gave the knife a good workout or cut a lot of cardboard.
For more serious sharpening I'm not sure what I would recommend as I don't have tons of experience with the others and wasn't super impressed with them as a beginner.