The best ones to watch if you're not going to watch them all are ones that don't push the main plot line along. For instance, my favorites are ones that feature Dale the Whale or an episode like Mr. Monk and Mrs. Monk but that's because they mean a lot to the continuing story.
I don't think I've seen an episode of Monk that I didn't like but these are some memorable ones that you can jump right into I think- Mr. Monk Goes to the Asylum, Mr. Monk Gets Stuck in Traffic, Mr. Monk and the Secret Santa, Mr. Monk Goes to Vegas.