What type of knife do you have? Is it a chef/cooking knife, a camping/survival knife or an EDC pocket knife? 

What is your skill level or profession? Do you enjoy the art of sharpening your knife or do you just want a tool to get the job done? 
 
What is your budget? Though most sharpeners are affordable (Less than $30). Some can cost a couple hundred dollars all the way up $1000+ 
 
These should all be considered before buying a knife. If you are just looking for some quick recommendations here are a few knife sharpeners that we think are great. 

Best EDC knife sharpener: Work Sharp Guided Field Sharpener, Gray - WSGFS221	Best knife sharpening stone set for professionals: Knife Sharpening Stone Set - 4 Grit 400/1000 3000/8000
Best general knife sharpener: ERTDDE Knife Sharpener, 4-Stage Chef Kitchen Knife Sharpener to Repair, Restore and Polish Knife Blades for Kitchen, Camping, Hiking	Best knife sharpener for professionals/knifemakers/hobbyist: TQ Manual Metal Fixed Angle Knife Sharpener Home Sharpener
Most affordable/cheap knife sharpener: 2-in-1 Kitchen Knife Accessories: 3-Stage Knife Sharpener Helps Repair, Restore and Polish Blades and Cut-Resistant Glove	Best knife sharpener for professionals/knifemakers/hobbyist: Chef'sChoice Trizor XV EdgeSelect Professional Electric Knife Straight and Serrated Knives Diamond Abrasives Patented Sharpening System, 3-stage, Gray

What is your skill level or profession? 

 This will be the biggest factor when deciding what knife sharpener to buy. Are you a professional chef who needs his/her high-end knives razor sharp for slicing and chopping your next gourmet meal? Or are you the outdoor enthusiast that carries a full tang or EDC pocketknife just in case. Or maybe you are just trying to get dinner made for your family and need a sharp knife that slices tomatoes instead of juicing them with your dull knife. 

 For the chef having a stone system may be your best option. It will give you a clean razor-sharp edge on any knife that you may have (besides serrated). The downside is that you will need to perfect the technique of sharpening a knife with stones. It also can be somewhat time consuming.

Knife Sharpening Stone Set - 4 Grit 400/1000 3000/8000

For the outdoor enthusiast having something easy and portable is most important. In this case having a small handheld sharpener that is budget friendly will be your best bet. You can achieve and sharp edge in a small amount of time and effort and get right back to your adventures. 

Work Sharp Guided Field Sharpener, Gray - WSGFS221

For the home chef having something quick and easy is of most importance. In this case an automatic sharpener or countertop sharpener would be a good choice. It is quick and easy so you can get back to cooking! 

2-in-1 Kitchen Knife Accessories: 3-Stage Knife Sharpener Helps Repair, Restore and Polish Blades and Cut-Resistant Glove

Chef'sChoice Trizor XV EdgeSelect Professional Electric Knife Straight and Serrated Knives Diamond Abrasives Patented Sharpening System, 3-stage, Gray

 What is your budget? 

Though most sharpeners are quite affordable. Some can also be very expensive. This is really where your profession or skill level comes into play. If you make knives or if you own many knives as a hobby. Then you might want to go for a more professional grade sharpener. One that gives you full control of angles, grit, etc. For the average user buying a professional grade system is not necessary. You can still achieve a sharp edge with a more budget friendly lower grade knife sharpener. If you are looking for something more professional grade that will give you a consistent edge, we recommend: 

TQ Manual Metal Fixed Angle Knife Sharpener Home Sharpener