Which writing system do you like the most?

I know Roman (English etc), Greek, Persian, Arabic (little bit), Gurumukhi (Punjabi) and Devanagari (Hindi).

I would say the best I like is that of Hindi. Because it is a phonetic script. That means you write what you speak or you speak what you write. There is ONLY ONE way to pronounce what's written so there are no confusions about spellings and pronunciations.

eg In English in cup and put "u" is pronounced differently. car and kar would be pronounced the same. British and Americans pronounce same spellings differently. But this is not possible in Hindi. So in all the writing systems I know, I find Devanagari to be the most accurate representation of spoken word.

Appearance or efficiency? I like the look of Medieval or Modern Greek, using lower case letters, but that is purely a personal esthetic choice.

For efficiency, each alphabet works well for the language it is designed for, but adaptations for other languages are seldom very good. That is why the Greeks introduced Cyrillic for the Slavs whom they were converting to Chrisitanity, because they thought they needed to make some really drastic innovations, such as separate symbols for palatalizing and non-palatalizing vowels.

The much more rigid Roman Church insisted on imposing its Latin alphabet on everybody. Even Latin derived languages had to use diacritics, accents and digraphs (awkward combinations) to use it. For the Germanic languages like English (apart from being unnecessary, we already had our Runic alphabet), it use makes it impossible to make fundamental semantic distinctions as between "poor man" when it means "pobre hombre" ou "pauvre homme" and when it means "hombre pobre" / "homme pauvre." The Muslims were almost as rigid in imposing Arab letters on Berber or Farsi, both of which had their own adequate writing systems before they were converted to Islam.

Japanese, however, really deserves a booby prize, for using a crazy combination of nomral Chinese symbols with syllabic adaptations of two distinctive varieties of Chinese cursive. But we must remember that writing systems, like formal grammars, and spelling conventions, are not concocted to make things easy, but deliberately as a social means to exclude the poor and uneducated.