This is an interesting discussion. At first I just wanted to blurt out what I thought were my favorite and least favorite traits, but as I thought about it, it turns out I don't really give my sims traits I like or dislike, but traits that I think most fit what I would image their personality would have been (as limited as that is in this game, on the account of, you know, sims having no actual personality). So due to some extra undesired free time as the result of a global event I tallied all the traits from each of the sims I ever made in CAS in the Sims 4 to see which ones I have used how many times. I usually delete all premade and townies that spawn and I've excluded any of the sims I downloaded from the gallery as well as the one child that has ever been born in any of my saved games and the two pets I made in CAS.
Toddlers ♂2♀0 = 2
Children ♂3 ♀3 = 6
Teens ♂31 ♀8 = 39
Young Adults ♂54 ♀8 = 62
Adults ♂7 ♀9 = 16
Elders ♂4 ♀6 = 10
= 135 total sims
As I suspected it turns out my most used trait is Active. 41/135 of my sims have that trait. Also, they are all male and with 7/41 being Teens, and 32/41 being Young Adults. Granted, I play with a disproportionate number of male sims in my games which also tend to be of the sporty or buff type, but I don't seem to associate the Active trait with female or elder sims.
My next most used trait is Romantic. 30/135 have that trait with only one being female (opposite sex relations are clearly a minority in my games, which probably explains the low birth rate).
My third most used trait is Geek, 24/135, two of which female.
Other traits I've used frequently (10-23/135) are: Bro (17 – exclusively used for male sims), Cheerful (12 – equal gender numbers), Creative (19 – most spread out among all age groups and fairly balanced among the two genders), Dance Machine (11 – apparently guys only again), Foodie (16 – apparently more associated by me with Adults and Elders than Teens and Young Adults, and the vast majority are females here), Good (15 – another fairly equal gender ratio), Loves Outdoors (11 – nothing particular about this one), Neat (17 – this one is fairly balanced gender ratio wise, but I've apparently given this trait to more female sims than male sims), Outgoing (13 – mostly male sims), Perfectionist (10 – again mostly male sims), and Self-Assured (19 – and again mostly male sims. Well, I do have a lot of those).
I can't help but wonder if I may have at least partly assigned traits based on a sim's gender, age, and possibly stereotypes.
If I look at traits that I've given to a majority of, or exclusively to, female sims I get: Cat Lover (1), Clumsy (1), Family-Oriented (5/7 Female), Cheerful, Foodie, and Neat (mentioned above), Insider (2/3), Jealous (4), Kleptomaniac (1), Slob (1), Snob (4/8).
Erratic, Hates Children, Paranoid, and Self-Absorbed are Traits I just haven't gotten around to use yet. With Hates Children being the surprise because I really thought I had given that trait to at least one elder. Turns out I just made him Evil and Mean. Unflirty is exactly the opposite of my playstyle in this game. I don't really like playing with children. I only ever made 6 in Cas and they're mainly world filler background NPCs so I don't see why I would want my Teens, (Young) Adults, and Elders to act Childish. I hate vegans, and Vegetarian is close enough so my Sims don't get to have that trait. They are all omnivores or just plain meat eaters in my sim worlds. I guess those last three are the traits I actively avoid in the game then. Although I'm also quite reluctant to increase the number of Kleptomaniacs from the one I have in my worlds now. I don't like being robbed, and neither do my sims.