I think my favorite banknote in my small collection is the Hong Kong Dollar - with four issuers (HSBC, Bank of China, and Standard Chartered for most denominations + HK Government for HKD 10 notes), there is such a lovely diversity of designs - on my last trip, I picked up a set of 100 HKD notes - two from HSBC, an older version BOC note, and a Standard Chartered note. I actually think my overall favorite note in my collection is the BOC note with the Bank of China Tower on the front and a view of Kowloon on the back.