I mean, what are we comparing here? Because on a technical level, it's the GBA, no question. It's basically a portable, beefed-up SNES, which is a generation of technology ahead of the NES. The GBA also benefits from a decade and a half(ish) of improvement in games design, so a lot of the games you'll see there outgrew the awkwardness of the 8-bit era. About the only thing you can knock it for is every iteration of hardware had something weird about it (the original didn't have a backlight, the SP didn't have a headphone jack, and the Micro didn't have the Gameboy/GBC backwards compatibility and wasn't compatible with a lot of GBA peripherals). So yeah, if you're talking about a new gamer who has never played either console evaluating which one is better and which they will have more fun with, there's really no credible argument for it not to be the GBA.
But if you're including historical and cultural relevance, that's where the discussion gets more interesting, as the NES was one of, if not THE most influential console of all time, and many of its games - Super Mario Bros., The Legend of Zelda, Metroid, etc. - not only kicked off some of gaming's longest running, best selling, and most recognizable franchises, but also established gaming design conventions that, in some cases, are still being used to this day. The GBA didn't have anywhere near that level of influence on the development of gaming as a medium.
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