I currently have a Corsair H100i Pro XT AIO inside my Lian Li A4 H2O ITX case. I don't really like how loud my fans are blowing and I'm thinking of going either liquid cool or air cooled with Noctua fans. I want something as quiet as possible and good at cooling as possible. Any suggestions?
I would say custom water cooling. It's not cost effective but it fits your requirements of being as quiet as possible with good cooling. Also you didn't say you dont want to sell your kidneys :P
Custom cooling isn't significantly better at cooling then high-end air coolers or AIOs, but since it so customizable you can scale it to your needs and make it inaudible. You could have lots of rads. Or if you want to keep a small case, you can have something like a large external rad (like a MORA) with large slow moving fans (silent).
Also it's mostly just initial costs that are high. Your AIOs pump won't last forever but you will have to replace the entire thing. You could invest in a better pump in a custom loop, and even if you have to replace it, you can reuse your fittings and rads, and some tubing.
Personally, I'd choose air cool. I have zero confidence about my DIY skills and I want to keep my PC simple.
As you mentioned
as quiet as possible
How much cost are you willing to pay? Bot only money, but also the time you'll spend on it. If you really love digging into it, then I'll suggest go ahead for liquid with caution.