For CPU, I rather like the XSPC Raystorm block for its high flow and built-in Acrylic LED slits; there are a few others that perform as well too.
For GPUs, there's enough options out there that are so similar I'd say just to pay attention to flow rates and then go for appearance. I say pay attention because some have a psi drop of 5+ at 1gpm while others are less than 0.8.
For RAM, decide on which RAM you will use first: there are quite a few water blocks that support Corsair's Dominator spreader screw placement, but GSkill gives you more options. You may end up needing to attach your own copper spreaders.
Be sure to pay attention to what motherboard blocks are available while choosing your mobo. There are generic blocks, but they usually interfere with GPU cards. If you're going to have one or more RAM blocks and more than one mobo block, you may consider puting some of them in parallel soas not to restrict your cpu and gpu block flow. Having half the flow rate through ram and mobo blocks will not matter much if at all.
If you want specifics, you'll probably need to post about what components you are going for.
Random things to keep in mind:
- It seems there has been quite a bit of shoddy nickel plating going around; though EK has been the most advertised, the problem is not limited to them. Do some research if you go for anything nickel-plated.
- If you use dye, pay attention to what coolants and additives are ok and which are not for that dye: there are too many compatibility issues to mention them all.
- Be sure to research the properties of your tubing: some will decay in UV light, some will drop plasticizer if you run certain coolants in them, etc.