My thoughts on the matter:

FFI:Temple of Chaos:

Very contrived. Not the most deadly dungeon by a solid one none-the-less. The first levels are unlinear which is always good, but the lower floors are just filler. There's a lot of variance though.

FFII:Pandemonium:

One of the hardest (but not the hardest in FF2) dungeons, the combo of the Jade Passage before Pandemonium will break your face in two. The designs and contrast between the two are also neat, and like most of FF2's dungeons there's a lot of false passages to deal with.

FFIII:Crystal Tower:

Nothing special, if you disregard Hidden Land Eureka which is techinally attached to the Crystal Tower. Apart from that it's just one bland looking designed floor after another. Until you get to the dark world that is...

FFIV:Crystal Palace:

One of my personal favs. Nice decor, especially for the last few floors. Not too difficult to nagivate, but certainly not linear. Lots to find considering how far into the game it is.

FFV:Cleft of Dimension:

Man, this place just rules. The completely different areas, the huge amount of monsters, FF's first "uber bosses". Probably my favourite.

FFVI:Kefka's Tower:

Extremely innovative with the switching between parties, but it can irritate you a bit later when you realise you need to backtrack to find a single switch you missed. Considering also the layout for some of the other areas of the game this dungeon looks atrocious in comparison.

FFVII:Northern Crater:

The first "unsquare" dungeon. Isn't as innovative as FF6 was, plus it doesn't really take advantage of the whole party splitting (you can't switch plus you can just re-enter that area once you reach the bottom). As for the actual layout, it's probably one of the best dungeons in FF7, if not the best.

FFVIII:Ultimecia's Castle:

As said previously, very creepy. For me it all felt a bit out of place, but for a slightly unnerving Gothic theme it does it's job. Again party switching comes into it heavily but not the most complex dungeon ever concieved.

FFIX:Memoria:

Art deco gone mad! This dungeon is completely whacked out. Twisting gravity, dimension warps. You name a cliche, chances are this dungeon's got it. Rather like the rest of FF9 in that regard.

FFX:Inside Sin:

Considering this is the first FF on the PS2, the last dungeon is terrible. Linear, badly designed and no where near a challenge. Granted it's supposed to fit inside of Sin, but still they could've taken liberties with physics. One of the worst.