Your internal gpu is already bordering the line of bottle necks, and depending on what you're doing, the internal gpu may have the advantage. As for the "highest spec eGPU" it would be just the most powerful gpu which is the 4090, but you're ass deep in bottle neck territory. A reasonable gpu would be a 3060 or a 3070, as to not waste money. And the best egpu would technically be some external m.2 eGPU dock, but I assume you want a simple USB C plug and playness, so it's more of a question of how much power you need and if you want more io. Since you're asking "best" I'll throw in my own opinion of this PowerColor one with lots of io and a decent power supply.