There are several factors to finding the right putter but the most important are:
Find one you can aim and one that you like the looks and feel of.
Others and still important are, the right length to fit your posture and a comfortable grip. Your stroke type and typical miss will also dictate whether you need a face balanced, or toe hang model. If your stroke is more straight back and through a face balanced putter will be best. If you swing the putter on more of an arc and misses are to the right (right-handed player) more toe hang will be better. Righthanders whose misses are left will usually want less toe hang. Toe hang is the amount the toe of the putter hangs down when balancing it with the shaft parallel to the ground. Face balanced putters have zero toe hang. Toe hang helps close or square the blade.
Cost might be an issue for many and don’t get hung up on the fact that the more you pay is better because there are a lot of good putters available under $250.
Yes AIM is important–it’s amazing using a laser how bad most people aim, even their own putter. We’ve found that right-handed golfers who are right eye dominant usually prefer or aim an off-set putter better. The opposite for right-handed, left eye dominant players–they need putters with no offset. If you can’t aim the putter correctly, how do you expect to ever get the ball started online.
Look and feel are almost as important as AIM. If you don’t like the way your putter looks or feels it probably won’t be in the bag very long.
Get a skilled fitter to help you determine what putters you can aim correctly and that you like their look and feel. Hopefully you’ll find 3-4 models that fit this. Take them out on the putting green and hit 10-12 3-4 footers with each, then the same with 25-30 footers. A good testament to the best one will be how many short ones you hole and how close the misses are, how you were able to consistently control the distance on the long putts and how well each looks and feels on the grass.
One last comment–a lot of folks comment they want something with forgiveness. If you’ve got a decent stroke and the right putter you won’t need that forgiveness.