I have found that the romance options that come with companions that you get as your first or second one are a lot more complicated and fleshed out then the ones that you don't get until your last companion, for obvious reasons. So that is one thing to keep in mind if you're looking at the different classes and the options they have.
That is a very good point.
In rough order of when you meet them (so descending order of how much content you're likely to get with them):
Corso, Jorgan, Vette, Mako, Kaliyo (starting companions for femSmug, femTrooper, mWar, mBH, mIA)
Elara, Kira, Quinn, Andronikos, Vector (Act 1 companions for mTrooper, mKnight, femWar, femInq, femIA)
That bunch definitely has the most fleshed-out romances, along with arguably Risha, who has some Act 1 conversations as a non party member.
Risha, Akaavi, Doc, DS Jaesa, Ashara, Torian (early Act 2 mSmug, mSmug, femKnight, mWar, mInq, femBH)
Iresso, Temple (late act 2 for femConsular and mIA)
Nadia Grell (for mConsular; you do meet her and [Flirt] before the very late point when she joins the crew and starts real conversations, but she still gets very little content compared to other love interests)