As the others have said, it's subjective but I think there are so many gorgeous things within the game itself.

I bought a few things but they only make up a small fraction of the outfits I've made, and most of the things I use regularly are completely made from in-game pieces. (At a quick count, maybe 10% of my outfits would require purchased items, and half of those are the cheap ex-event pieces - some of which I did get from the events.)

MS items tend to have specific looks that are hard to replicate with in-game items, but they're not necessarily any better.

A lot of the time, it's better to mix and match parts of different sets as it makes for much nicer outfits than putting all the pieces of a set together. This goes for both in-game and bought items, but a lot of the bought ones are full-length so they're less customisable.

If you want inspiration, go browse Eorzea Collection:
https://ffxiv.eorzeacollection.com/glamours