It's sorta tricky to make too many generalizations about Syd as a songwriter because his solo stuff is fairly different from his Floyd stuff but first of all, this idea that compositional simplicity is inherently inferior is bullshit. The fact that many of his songs feature only a few simple chords doesn't detract from his songwriting ability one iota.
But anyway, I consider him my favorite songwriter because of the uniqueness of his voice as a songwriter. As simple as they are, nobody wrote songs like him- the uniqueness of his compositions were praised from the time of the earliest Floyd singles. And as a lyricist, what really appeals to me is the balance he finds between vulnerability and surrealistic poeticism, the 2 main qualities I look for in lyrics. His lyrics also display his visual artist's eye in their uniquely evocative quality (same with Beefheart, another one of my favorite lyricists though he obviously uses a fairly different approach from Syd.