This is the alpha and omega of the kitchen. Ask a chef — or a chef-aspiring obsessive amateur, or a casually really excellent cook, or a culinary professional in any branch of the industry — what their numero uno kitchen tool is, and they will say “my knife.” You want one with a sharp, strong blade, which can come one of two ways: You can pay a large amount of money for a high-carbon stainless steel blade, and have that knife forever, or you can pay a moderate amount of money for a ceramic blade, and have to replace it frequently.
The length of your blade (they start at about 6 inches and go up to 9 or so) says nothing about who you are as a person, or how manly you are. All it says is something ergonomically complex about the size of your hand and the way you comfortably hold things. Don’t buy a knife online. Go to a real store with a good knife selection and try a few on: hold them, heft them, practice chopping with them. Once you find your knife, learn how to hone it, learn how to sharpen it, learn what the difference between honing and sharpening is. Take a knife skills course. Nonchalantly show off your garlic-slicing prowess in front of your friends and be all like yeah, whatever.
My pick: I have a Wusthof Classic 8-inch Cook’s Knife. I swear that when I bought mine in 2005 they called it a chef’s knife. I’m also curious now if the more-general designation as a “cook’s knife” is to distinguish it from knives intended for e.g. murderers.