I think you should find games first. No point in owning an NES without finding something to play on it. But if you are looking for opinion on the hardware itself.

I own all 3 systems.

Atari 7800: isn’t fare to compare it to a NES or Master System. Think of it as a suped up 2600. Great system if you use the European controller or a Sega 9pin controller. Game selection is good but limited. Best way to play 2600 games. Rf only sucks but can be modded. I’d get it if there are 2600 or 7800 games you wanna play. But the Atari 50th collection has pretty much everything you’d want to play on it.

NES: is a good system for the time, but hasn’t aged well. Getting games to work has turned into a ritual for me. But when they work you’ll have access to the best games from the late 80’s and early 90’s. Controller has aged like fine wine. Most of its non-nintendo library can be found elsewhere for cheap while the Nintendo games are behind a subscription service. Not many games exclusive these days but worth playing anyways

Master System: the NES we have at home. Mostly a similar library of games to the NES but are sometimes better, sometimes worse. Depends on the publisher. In north america it was forgotten and game selection was limited. Controller is mushy but a genesis controller can fix that.

Out of the 3 the master system is probably the best experience, but the NES has the best games.