Probably get a build with a Core i9 12900K, and a 3090 Ti Since you want a custom prebuilt, the Origin Corsair 5000D is the best option(not the best value, but the best performance). Customize it to the i9 12900K or KS(if you want the absolute top), and get the RTX 3090 Ti as well for around $5000.
Below is a link for a sample build with a 12900KS, a 3090 Ti Hybrid, and possibly everything else you would need for something this high-end. This build includes an 360mm AIO liquid cooler, a total of 6TB of NVMe SSD storage and 64GB of ram. If you wanted to go custom water cooling, you would only need to swap out a few parts, but as of this posts, there is no water blocks available for the 3090 ti's.
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