I'm OK with any companion being highlighted, especially those romeance-able.
So far I've gone thru all republic side storylines plus the Agent, and I'm about to begin Alderaan on Sith Warior.
Trooper: of all the Havoc members, I've found Aric most interesting, and M1-4X most hilarious in his ultrapatriotism. Elara Dorne was kinda too "rules above all" for my liking and Tanno Vik downright not fitting. I guess I share many viepoints of Aric myself... Yuun is just ... alien.
Smuggler: Corso is ok as a co-pilot, but it is in the romance side where he shines if you happen to be female. Bowdaar is a Wookie, and that is about all about him. Risha is great , multifaceted character, and her romance is nice too. Akaavi is somewhat dull to me, but if you like our romance armored, head on her side, but watch out for possible catfights over you... Guss is funniest crew member out there. I've spent most of the later storyline hanging out with him and trolling enemy NPCs.
Knight: T7 is better Jedi than my character! but of course not exactly a romance material... On the other hand Kira is highly unorthodox for a Jedi - and very fun to run around with if you like her sharp wit. Doc is fun too, especially when you get to know his REAL name. Sgt.Rusk is so bland I barely remember him. Lord Scourge is quite interesting in his Sith ways, but unless youre total darksider you'll end up clashing with him often.
Consular: Qyuzen is very interesting with his alien hunter code and honorable view of the world. I ended up respecting him much. Tharan is total tech-otaku, down to his relationship with Holiday. Zenith is quite a hitman in his mentality of no holds barred guerilla sniper, but definitely oce you gain his trust it gets better. Iresso is coll nice soldier, and definitely has one of better backstories. Nadia finally is a bit naive, but her dedication to you makes her a really sweet romance.
Agent: Kaliyo is a bombshell, love her or hate - you cant ignore. I personally ended likeing her extremely well.
Vector seemed a little bland to me, but his diplomatic ways are perfect counterbalance if you dont like Kaliyo's explosive ways. I spent much of the later game with Lokin because his healing effectivenes matched his cold analytical mindset similar to that of of my agent, with only occasional softness of her lightside making him a bit unsettled. Ensign Temple was not far from my character - imperial loyalist with conscience. I practically never went out with SCORPIO as she was just a bit too machinelike for my taste.
Sith Warrior (until lvl 25 so far): Vette is second coming of Mission Vao, from the very start amusing and likeable. As for Quinn, I found teasing him with flirting extreme fun (my SW is female), and expect it to develop into great romance.