It's no secret that PEBMAC passionately recommends 1Password ("Hi nice to meet you have you heard about 1Password no pull up a seat..."), but you may be wondering why 1Password gets all our praise when macOS comes with the handy-dandy iCloud Keychain, which is free? The answer is a bit verbose, so... well, pull up a seat.
Cross-platform support: 1Password works with Macs, PCs, Linux, iOS, and Android. iCloud keychain only works with Macs and iOS.
Support for other browsers: 1Password has browser extensions for Safari, Chrome, Firefox, and Opera. iCloud Keychain only supports Safari.
Store other data types: One area where iCloud Keychain has an advantage is for storing wi-fi passwords and syncing them between devices; so if you join a network on your phone your Mac can automatically connect to the same network (this is the only reason we use iCloud Keychain in addition to 1Password). However if you want to store things other than passwords or system items like security certificates, then 1Password is the clear winner with the ability to store notes, images, passwords, identities, credit cards, memberships, and pretty much anything else you can imagine.
Credit Cards: While it's true that iCloud Keychain can store credit card information, Apple won't let it store the CVV code that needs to go along with it. Apple claims this is for security, but the fact that it also encourages users to use Apple Pay can't be a coincidence. 1Password can store everything you need to use your credit card, up to and including the telephone number for your issuing bank should you choose to store it (which will be really handy if you lose your card and need to call the bank to cancel it!).
Time-based One-Time Passwords (TOTP): If you use a service that requires a time-based password you know what a hassle it can be. 1Password supports these ever-changing passwords; iCloud Keychain does not.
Multiple Syncing Methods: As its name implies, iCloud Keychain requires iCloud to store and sync your data. 1Password gives you a multitude of options, ranging from storing it locally so that you're in complete control of your data at all times, to syncing through popular services such as Dropbox (and iCloud).
User-Friendliness: 1Password does have a bit of a learning curve, but once you get used to it it's very easy to find what you're looking for. On the other hand, Keychain Access (the app on macOS that lets you access everything stored in your keychain) is about as user-friendly as a pet porcupine.
Sharing: If you believe the maxim that caring is sharing, then 1Password is your best friend, whereas iCloud Keychain hates you and everyone you love. 1Password offers everything from sending a login as an encrypted text message all the way to using 1Password for Teams, an optional paid service that gives you the ability to grant, revoke, and otherwise manage sharing passwords with others. iCloud Keydchain will require you to find the item you're looking for (which itself can be mystifying) and then copy and paste the relevant data into a possibly-insecure communication method such as email.
In the end, the thing we like the most about 1Password is that it allows users to manage their passwords in a safe and secure way, giving them the ability to use the complicated and unique passwords that are becoming increasingly important (hacked passwords are like failing hard drives: it isn't a matter of if it's happened to you, it's a matter of when).