An article on Wikipedia defines powerpop as being "characterized by strong melodies, crisp vocal harmonies, economical arrangements and prominent guitar riffs. Instrumental solos are kept to a minimum, and blues elements are largely downplayed." That's an excellent description, and Bronco Bullfrog are a perfect example of a great powerpop band.
The same article goes on to credit Pete Townshend with coining the term 'power pop' in a 1967 interview. Bronco Bullfrog wear their influences proudly on their sleeves - The Who, The Beatles, The Byrds, and just about every other class 60s band you can mention - but they don't sound like imitators. They just sound like a band who took a wrong turn in their time machine. If they'd been around in the 60s I'm certain they'd have hit the bigtime - but then again, I can't work out why they didn't have better success in this day and age.
'Oak Apple Day' is my personal favourite of their albums, but I recommend them all. The band is just as at home with a haunting ballad as they are with an uptempo romp or a psychedelic hippie-trip. Highlights for me on this album are the pedal steel on 'Sunday Wheeling', and the fabulously tight lyrics of 'Wrong Things Right' - a bouncy song about romance gone wrong.
Unfortunately Bronco Bullfrog have gone the way of too many great bands and split up. Try Heyday Mail Order, Not Lame, CD Baby, Amazon, and eBay for their albums - and be aware that they are nothing to do with the British film of the same name!
