1983 CB1000 Custom: regret selling it.
2000 Buell S3T: regret buying it. damned sexy looking bike though.
2001 RC51: Regret converting it to full track and selling off all the street parts. Bike was a serious blast (and a workout) to ride. Forced me to improve my skills.
2003 600RR: Regret polishing the bolts and not clearcoating them. They developed surface rust and caused the guy who purchased it from me to replace them.
2005 BMW R1200GS - not a single regret, most useable bike I can imagine having. No modifications except the exhaust and it sounds sweet, Leo Vince titanium slipon.