Too many series well executed and designed during these past years. It is to hard to choose, especially since the OP didn’t specify a year. It would be easier that instead of “The most beautiful designed anime”, the topic title was “The most beautiful designed anime of 2007“ or so.
Some I find the most beautiful:
- Hachimitsu to Clover (2005). Technically and artistically, it one of the most beautiful series made during 2005. Beautiful crafted backgrounds with a soft palette of pastel colors gave it an unique feel. Character designs, even kinda bland, had unique and interesting set of personalities that make them shine during the whole series. Their animations were done with care in detail (facial and physical). All this would been nothing (mere eye candy), if it wasn’t for the extremely well executed plot; a touching and compelling story about a group of art students as they go thru the ups and downs of college and their life. In the end, Hachimitsu no Clover is one of the most beautiful series of 2005.
- Code Geass (2006-2007). Nicely well done. Esthetically there isn’t much to say. Surroundings and character designs are detailed and unique (mainly the main characters). Motion is unquestionably fluid (mecha and human), especially in fight scenes. Characters retain their proportions all the time. However, what makes this series one of the best of 2006-2007 is its well written script. Characters interactions and desires as well the amount of twists (WTF moments) the storyline presents.