Mario: "Forget about General MIDI (GM) sound sources. The best sounds come from non-GM synths."

I do not fully agree. I find Halion and Sonic are quite capable GM synths at 20+GB of sounds, especially as "all in one solution" for the price point. All drumsets are adjustable-per-drum and many many other features. Both items come in one package and are usually 1/2 off at least once a year.

Handpicking, there are always SF2 items which are either free or "cheap enough" some of them might not be as tweakable as Kontakt instruments but there are some very decent and realistic ones.

Do not get me wrong, I like Kontakt and it's amazing instruments, but I feel that some titles are way overpriced and unfortunately no GM.

Bottom line,

Kontakt by itself comes with pretty weak "factory" library. To get it right to "realistic specs", you have to hunt for sales/upgrades and by the end of the day, it will cost quite a lot, but you can achieve the "better" sound quality.

Halion / Sonic on the other hand "As Is" But the included library is very solid. Nice selection of traditional instruments. Plus GM. Very reasonable when on sale.

And of course #3
Compile VST's / SF2s etc. from various companies to make something more personal.