I recommend Undermountain
Okay, well, I'm biased because I don't know Rappan Athuk. However, while ToEE is very large, I think Undermountain takes the cake. The original box set, which I have, has four poster size maps, which are supposed to detail 3 levels out of (I believe) 9 main levels and countless sub-levels. There are, as I recall, roughly 30-60 rooms detailed on each level, but there are far, far more unlabelled and empty rooms for you to detail yourself. Like I said, four poster sized maps of dungeon. And, if you have the money and inclination, you can get the Undermountain II boxed set for even more maps. I wouldn't recommend any of the other supplements, though, for your purposes. They are highly detailed, and using the maps without the accompanying text is probably not worth the effort, as each has only two 8x11 maps.
And, for the curious, the halls of Undermountain were originally dug by dwarves, duergar if memory serves me correctly. In the FR storyline, they were then taken over by drow until the mad mage Halather came along to claim them for himself, but you could obviously change all of that.
Anyway, that's my 2 cp.