The best water cooler alright put together no need to fill, would be a water cooler that is 100% copper radiator and plate, which only a couple of water coolers have those specs, and are priced at 170-220$ Out of them all the cooler master 240L would be the best, in my opinion, because look at cooler master's other products, the v6 heat sink or v8, or even the smaller water cooling products, they all have for the most part very good reviews. Cooler master productions are known for great working nice quality items for a cheap price from cases, case fans, heat pipe heat sinks to water cooling heat sinks. Personally im going to buy this 240L cooler master water cooler soon, and i have looked at all my options and have realized this is the best product out there for the price, if i was to go lower i would buy the h100i corsair but that is even close to the 240L because its an aluminium radiator.
Also remember the 240L is new on the market so there will be more and more reviews coming out and threads, so my opinion is that if your going to buy a water cooler wait a little and verify.