My list of priorities on a bike...
- Frame
- Wheels (includes hubs, spokes, rims)
- Fork (includes headset)
- Drivetrain components (includes shifters, gears, derailleurs, cranks, BB)
- Other components (saddle, stem, handlebar, grips, pedals, etc...)
I based this list on what's the least easily and economically feasable to change/replace... in other words, what you're locked into when buying the bike.