I just bought a powerquest 280 sc was curious what speakers fit in the factory holes and have the best sound? thanks!

Kind of doubt you'll find one that fits the old opening, and you probably want to go somewhat bigger ayway. On my 257, the holes were a puny 5.75 inches - no 6" speaker I looked at would fit. Since I was grinding anyway, I went with MTX Thunder Marine 7.7s driven with a 90W amp.
http://www.mtx.com/marine/
Sound great, even w/o a sub and ~100/pair from amazon. I did have to put Dynamat around the rear speakers, since the panels were vibrating fairly bad - but that is the boat construction .