I've always really liked Nomura's Advent Children designs. Cloud's redesign in particular had a really interesting silhouette, and I think Tifa's was actually superior to her VII design.
Other designs I liked: Balthier, Squall and Vivi come to mind. FFXIV also has consistently good character designs.
Those I like less: Tidus' design is a little too busy, and the silhouette is a little too boxy; I like his Dissidia design more. Sazh has kind of a ridiculous design (the hair chocobo), despite being by far the best character in XIII. Zell has a pretty obnoxious design, but I think that's also consistent with his characterization, so I'm not sure if the design itself is actually poor.