OK, so we only get to take a small number of traits, that remain fairly fixed in the game, while we also get to take more and more lifetime rewards as the game goes on. Some of those rewards have effects in the same categories as traits, so I think it makes sense to only pick traits with effects that aren't available via rewards.
When it comes to gaining romantic interests, let's first consider the relevant lifetime rewards:
Attractive (15,000) meeting sims start at higher relationship level
Above Reproach (35,000) reputation unaffected by being seen cheating
Clean Slate (25,000) resets reputation to spotless
Eye Candy (5,000) +10 mood to nearby sims
Master of Seduction (15,000) romantic attempts always succeed
No Jealousy (10,000) no drama when seen with other romantic interests
Stone Hearted (10,000) no sadness after breakups and major events
So, I think one of the key rewards that changes the importance of traits is Master of Seduction, since it guarantees successful romantic attempts. Second place is probably Attractive, which starts you at a higher friendship level and thus you'll need less time to raise the bar from there to romantic interest.
There are also Charisma skill challenges that impact romance:
Celebrity -- relationship bar increases faster
Personable -- learn other sims’ traits faster
Super Friendly -- relationships never decay
Everybody’s Best Friend -- skips Good Friend and becomes Best Friend
Comedian -- Jokes always succeed
Here are all the traits I'm aware of that might impact romantic interests:
Charismatic -- Gains Charisma skill more quickly, others more likely to listen, never bore Sims, makes friends quickly
Friendly -- Friendly social interactions are accepted more often, good listener, even when others repeat themselves
Flirty -- Flirt with other Sims early in a relationship, flirts will be accepted much more often, gifted at giving massages
Good Sense of Humor -- Sims laugh at your jokes more often, jokes strengthen relationships more than normal
Great Kisser -- Your Sim’s kisses will have a greater effect
Hopeless Romantic -- More receptive to your advances, better mood around romantic interests
Inappropriate? -- Your Sim can try to kiss, tickle, and even propose marriage when other Sims wouldn't dare
Irresistible -- Gain relationship faster, rarely have dates rejected, sims want to socialize with your Sim more often
Given that Master of Seduction guarantees that romantic attempts always succeed, it seems the best traits then are either ones like Charismatic and Irresistible that increase the rate of relationship gain to allow an attempt, or traits that allow romantic advances at a lower relationship level like Flirty and Inappropriate.
I would tend to favor the Charismatic/Irresistible option, since they have additional benefits outside of romance. I am curious if I had to pick between them, which one is better?