I went with Banjo Kazooie. But I'll admit, I've only played it and Conker on the list. I've even played and beaten BK twice, but still haven't gotten around to Tooie... hopefully one day.
Conker had tons of humor and was definitely worthy of the Mature badge, but it seemed a bit gimmicky / contrived at the time. Still fun, but didn't have the lasting appeal that BK did.
The levels in BK were so varied, and the Mumbo transformations were awesome. Being the pumpkin at the spooky house is just funny, and being the alligator in the swamp was just plain fun. These were varied enough to keep the gameplay fresh and interesting without becoming too repetitive. And for those of you completionist, plenty of jingos / notes to collect.
Great game and I loved it just as much as Mario 64.