I may not be the most prolific of Sim-creators out there, but I thought I'd chip in with my two cents. Although I will try not to repeat what has already been said, I do agree wholeheartedly that sliders are your friend when it comes to making unique-looking Sims. The two Sims in the collage below are using the exact same CC hair, yet they look completely different. It's all thanks to the magic of sliders :) Also note that no CC sliders were used in creating these guys (except for one that adjusts brow thickness).
https://i.imgur.com/2Fmtjny.jpg
On to my second point: I believe that it's more than just facial features that contribute to the overall look of a Sim. I find hair and style of dress to be important distinguishing features as well. Take a look at these guys, for example. I know they're not Sims (they're from another game I love called Stardew Valley) but I think they're a pretty good example of effective character design. Not only do they all look truly distinct from each other, each of them also gives off a specific impression. For example, the use of bright colours in Sam's design gives the image of a carefree young man, and his flamboyant, spi'ky (why the hell does that word get censored) hairdo could signify a rebellious nature. Where facial features are concerned, Harvey's rounder jawline and softer nose could indicate that he's a bit of a softie.
https://i.imgur.com/YKl76a9.jpg
Of course, the best way to learn is by doing! And when you Simmify fictional characters such as these, you really start to get an appreciation of their design :) I've definitely played with the sliders here, but I also did my best to use clothes and hair to capture the unique feel of each character.
https://i.imgur.com/XfW2SHj.jpg
Finally, on the topic of CC sliders: I do think the base-game sliders already give you quite a lot of options. Though if you're looking to start using some CC sliders, I would recommend getting a few custom sliders for the jaw. I have recently begun to use a few of these, not because I felt EA's jaw sliders were limited, but rather because they weren't very user-friendly. And if you want more custom sliders, only add a few at a time! I remember once downloading a whole heap of CC sliders and then feeling so overwhelmed with all the new options! The new jaw sliders worked a treat though, and without them I don't think I would've been able to capture Caleb Vatore here :)
https://i.imgur.com/2efyPhT.jpg
(Yes, I developed a crush on a plasticine vampire whose hair looks like someone glued a bunch of leaves to his head. Sue me.)
