First let me say I think a Pac vs Floyd fight, at this stage in their career,(past prime) is a 50/50 based purely on whoever has the best night. Now that, that is out of the way. I was watching Kell Brook fights against small opponents and noticed he does a good job keeping little guys at range, and shooting his counter. He's pretty strong and has good timing on his punches. Yes he has vulnerable spots, but at this point is Pac's career I feel Kell Brook is a 60/40 favorite in which makes him the most dangerous fight for Pac. The Kell Brook vs Khan fight will be exciting because we get to see how he fair against rangier opponents Heres some bias highlights of his range against Porter https://www.youtube.com/watch?v=CaOEvOCDXrw
Pacquiao's most dangerous opponents at 147 are Mayweather and Marquez. Beyond them, next up might likely be Brook.
If Brook decides to make it a boxing match without being as consistently aggressive as he's supposed to be then Khan would definitely outbox him.