When I say '06, I mean stuff that aired or was released (in the case of OVA's and movies) in Japan during 2006.

Anyway, before I say what was most disappointing, I'll mention animes I started watching during the year but just stopped watching for whatever reason:

Yoake Mae Yori Ruriiro Na (so achingly dull)
Shounen Onmyouji (I was expecting maybe some shounen-ai, but no such luck - too bad, it had pretty animation)
Tokimeki Memorial~Only Love~ (besides the chick [as in, baby chicken] in the first episode, it just wasn't funny enough)
Death Note (its good, but it just didn't interest me enough to keep watching)
Negima!? (it's ok, the character designs were wicked cute, but I was watching a bunch of better stuff already)
Code Geass (creepy over-the-top nationalism + ripping-off-a-Gundam-series-I-didn't-like-to-begin-with = why bother?)
Otome wa Boku ni Koishiteru (didn't grab me)
Suzumiya Haruhi no Yuuutsu (in this case, I think its because I watched it in correct chronological order - after seeing the final aired episode in its correct slot of episode 5, I didn't really care any more, even though I did find the characters and animation fairly enjoyable)
Cluster Edge (only included because I continued to sporadically watch episodes into early 2006)
Kashimashi (technically an '05 title, but it continued into '06)
High School Girls (manga was way better - the fanservice here was just creepy/weird give that this thing was supposedly aimed at teenaged females)
Inukami (decent until it dove into harem territory... although the guys-possessed-by-dogs episode was way weird; I dropped the moment I saw a maid)

As for which was the worst... well, I'm a little up in the air - I wanna say Cluster Edge, but that was technically a 2005 offering. Cluster Edge, though, was definitely the most disappointing, even considering its being a carry-over from '05. It had great potential - cool planes, pretty characters, potential shounen-ai/yaoi... but it just wasted it by being ridiculously boring and having flat, mostly annoying characters - especially the main one. I still am amazed that I managed to drag myself through sixteen episodes of this... and also sad because I know I'll NEVER get those eight hours of my life back!

As for worst, I guess it's a toss up between YoakeNa and Inukami. YoakeNa was just so insanely boring I'm surprised I survived two episodes (I actually was gonna even give it three, but then some creepy old guy kept bursting from under one of the girl's skirts which was just too much for me) - the only thing I liked about it were the female school uniforms. Inukami wasn't boring, really, but it just had too many weird bits and fanservice - in addition to it deciding to go the tired harem route. Too bad, 'cause Youko (ugh, wtf about the name) was pretty cute.