Recently put together my first custom loop, and I pretty much did all the research here - these forums are full of valuable information. Everything in this thread so far is good stuff, too. My advice would be to research each individual component, and don't forget the "clubs" (i.e. EK Club). Yes there's thousands of posts to read but with this kind of investment, it's worth it.
To answer your question directly, I'd have to say in most cases there's no clear cut "best" in terms of performance. A water block is a water block, it may or may not crack. Same with a reservoir. A fitting is a fitting, it may or may not leak.
With that said, these were my choices, keeping in mind that my priority was making it as compact as possible:
CPU + Pump Combo: Swiftech Apogee Drive II
The pump in this unit is the Swiftech MCP35X PWM. It's noisy at full speed, but you can slow it down and still maintain good flow.
GPU: EK
Reservoir: EK
Not much to say except these were in stock at my local Microcenter and I love buying stuff there.
Radiator: DazMode Darkside
If space is a concern for you, these are one of the slimmest and most compact radiators in the market.
Tubing: PrimoChill PrimoFlex Advanced LRT
You can hardly find testimonials on any other tubing because these are so widely used.
Main Fittings: Swiftech Lok-Seal
Bitspower fittings are very popular for their aesthetics, but these Swiftech Lok-Seals were available at Microcenter so I went with them. No fancy colors, but they're half the price of BP and I haven't had any issues with them so far.
Adapter Fittings: EK, Koolance
Rotary fittings, valves, etc. Only difference between brands is aesthetics IMO.
Quick Disconnects: Koolance
Koolance QD3s are a favorite here.
Coolant: Mayhems Pastel
I technically bought EK coolant, but they're essentially repackaged Mayhems. The colors look great and all the necessary biocide/inhibitors are already mixed in. Just add distilled water! Check the Mayhems Club for some wicked coloring ideas.