Good questions!
As for the types mentioned, I enjoy all of them. A game with a well told romance story can be incredibly enjoyable, whether it turns out to be a happy ending or a tragic one. Personally, the only thing I would really like to see is something to show for your choice at the end. If your character goes out of their way to romance and marry a character, that shouldn't just be tossed out the window come the closing of the curtains.
As for game mechanics, there definitely should be dialogue and some quests I think. Cut scenes are enjoyable, but the work required to make them might not be worth it (rendering a custom made character and the other, especially if there are multiple NPCs to romance). As mentioned above, something added into the game ending I think to show your choice (whether to romance or not) should have some impact.
As for ones I've enjoyed, the ones that come immediately to mind are Bastilla from KotOR and Raven from Arcanum. The later especially I believe a good example since the ending does give mention to what happens to the both of you (even if a bit generic). Likely others as well, but they are currently slipping my mind.
The last question is quite subjective based on who you ask and I would require some time and much more sleep then I have currently to answer it completely.