Truthfully, I'm of the opinion that a lot of the NES's library hasn't aged super well if you didn't grow up on them, and many of the best classics have better remakes elsewhere. There are of course some gems worth playing, but if you can't get into something like Zelda 1 I think you're going to find a lot of recommendations are kinda downhill from there.
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Super Mario Bros 1, 2, 3 - Obviously. For 1 and 2 (and 2j, but idk if I should be recommending that one here) I prefer NES, but 3 I actually like SNES better.
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Kirby's Adventure - Arguably the most technically impressive NES game, pushes the limits of the hardware super hard. Only knock against it is that said pushing does result in some lag. So you may prefer the GBA remake, or the 3DS 3D Classics version which faithfully reproduces everything but the lag.
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Mike Tyson's Punch-Out!! - My absolute favorite NES game, but its brutal difficulty curve will be an acquired taste. If you can get into it I think it holds up well.
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Wario's Woods - Fantastic versus puzzler if you have a friend to play with. If you don't, the SNES version has a vs CPU mode. The singleplayer is solid too, but I really love the versus.
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Dr. Mario - I'd be remiss not to mention this here since I recently made a small video essay about the game, but I have to attach the disclaimer that I actually think NES is the weakest version. Like Wario's Woods, there's no vs CPU in this version. NES isn't bad if you have someone to play with, and the tournament scene actually prefers it so maybe don't listen to me at all, but I'm partial to 64.
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Celeste Mario's Zap & Dash - If you want to try some romhacks, here's SMB1 turned into a Metroidvania with Celeste mechanics ported in. I think what impresses me the most is that they got 4-directional scrolling into this engine.