I roleplay easiest in Morrowind. There are a lot of reasons for this. Foremost among them, I think, is the Topics and hyperlink dialogue system. It is a very conceptual approach to dialogue, and it allows me to use my imagination to its utmost. A game has to allow me space to use my imagination or I cannot roleplay. If a game attempts to hold my hand and do everything for me, I cannot roleplay.
When my character's dialogue is written out explicitly, I rarely feel that the developer has written what my character would say. But this is to be expected. The developer doesn't know my character. How could the developer possibly write dialogue for my character? They can't.
Each time I click on a hyperlink with a different character I imagine my character speaking a different line of dialogue, one that is unique to that character. If I click on that hyperlink fifty time with fifty characters I can imagine fifty different lines of dialogue.
The fact that is is almost entirely text dialogue also helps me. I "hear" Sellus Gravius' dialogue in different tones of voice each time I play a different character. One time I might imagine him speaking the lines in a serious tone of voice, another time I hear him speaking the same lines in a sarcastic tone of voice, another time I hear him speaking the lines in a kind of bored, bureaucratic tone of voice.
Even the fact that the NPCs have no schedule helps, oddly enough. It allows me to imagine lives for them. When I leave Seyda Neen I imagine Eldafire and Fargoth and Arrille and all the others getting into all sorts of adventures while I'm away. I imagine lives for them. That is harder to do in the later games.
I guess what it all boils down to is that the bare bones nature of so much of the game allows me to use my imagination. When a game supplies voice acting, schedules, and other technical bells and whistles in an attempt to simulate "life" it inadvertently removes some of my ability to use my imagination.
I'm a little like a kid who plays with the box rather than the toy. The box allows me to use my imagination to its fullest. Morrowind, to me, is a great box to play with.