I find I like very few of his original songs. Especially in the old days, he just didn't have the musical talent to make an interesting song from scratch. Too many have repetitive, jokeless choruses that are repeated too often, and almost none of the songs do anything interesting musically. Occasionally you get a gem like Hardware Store, but for the most part, his originals bore me, and even ones I like at first age poorly. Also, although several of his sound-alikes are spot on, some just totally miss the mark. For instance, Skipper Dan fails to capture the essence of Weezer, despite trying too hard to emulate Pork and Beans.
For me, the parodies are definitely the draw of Weird Al. The more sanctimonious or serious the original song is, the better.