Another vote for "Won't Get Fooled Again". Also, "My Generation" or almost anything he did. He was very unique in that he didn't really keep the beat, but almost soloed constantly. John Entwhistle and Pete Townsend kept stricter time than Keith did.
I used to think that Bonham was overrated until I listened further and realized that some of his stuff was HARD to play. He used to work as a brickmason and had phenomenal leg strength which allowed those bass drum triplets and heavy grooves. The shuffle he plays on "Fool In The Rain" is amazing.