Best Power cable around/prefer under $300....

For under $200 you won't beat an Audio Envy power cord IMO.

https://audioenvy.com/product/power-cord/5732/

I’m a big fan of Empirical Design power cables—the ED-416 would fall within your budget. I’ve been using Karl’s cables for over a decade. So many component changes over that time—same power cables. The ED IC’s are also great, reasonably priced cables—still using them after many years as well. FWIW, these are the only cables I’m aware of that won the TAS award two years in a row.

I have a Monsoon on my PrimaLuna integrated. I could easily hear a difference I liked, more tone, body and detail. You must do a home demo to decide for yourself

Go with Shunyata for sure.

You are right, they are all much better than your freebie rubber power cord. Anything, absolutely anything is better than that.  

All those other cable suggestions are probably good. There is however only one in your price range that has stood the test of time- Synergistic Research Master Coupler. Originally around $400 back in the 1990's they are still made today and come up used for anywhere from $200 on up. This makes it probably the only one mentioned you could buy and use several years then sell and not lose money.