That's what title says. I also looking reliable and can running 24/7 stay on, and fits this $750 to $775 PC build budget.
PCPartPicker part list / Price breakdown by merchant
The most sensible option in my opinion would be this Corsair RMx.
If you want a longer warranty, the Seasonic Prime Gold units are at the upper end of your budget.
The be quiet! Straight Power 11 may be worth looking at due to its internal design and components used, but performance testing puts current performance under the other two options and long term it has a shorter warranty than the Seasonic Prime Gold.
People put too much importance on PSU performance. If it's within ATX specification, it's good. Sure, the lower the better, but one should look at internal design and build quality first and foremost.
Straight Power 11 I would put over the RMx due to its quad 12v rail design, which is safer, when implemented properly, than a 12v design.
RMx uses a single rail design which I don't personally like but I have no complaints of the unit otherwise.
The Primes are really good but are notably more expensive.
I find Corsair RMx 550x (2018 version) for $85 an Amazon... I watched Corsair RMx review on YouTube, and I just noticed that modular cables too stiff. Any way to lossy it up?
Nope. That's just the result of high grade cables and thick insulation. The RMx's quality is excellent, and I would definitively recommend it.
Okay, good to know. That RMx is almost perfect for me. I have another modular cables question. Can I buy different colors than original like Corsair's Premium Individually Sleeved PSU Cables Pro Kit or CABLEMOD.com?
Cablemode PSU cables kits are compatible with all the following PSUs
So there shouldn't be any problems, as long as the PSU in question is present on that list