1) All copper water blocks with soldered on barbs/connectors.
2) No res, get rid of it.
3) As little hose/tubing/piping as possible.
4) High quality pump.
5) High quality rad fan, Panaflow comes to mind.
6) Quality hose clamps, probably of the metal variety.
If that leaks or fails then your to stupid for water cooling and need to invest in a good air cooler.
