As far as New Vegas mods go, you can't go wrong with:
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NMC's Texture Pack: Improves the textures of almost everything in the game, and has options from having the same resolution as vanilla textures to HD, so you don't have to experience a drop in quality;
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New Vegas Anti Crash: I never had a lot of crashes when playing but this mod brought that number down to a definite zero, good for anyone.
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Wasteland Flora Overhaul: Improves the look and distribution of plants in the Mojave, with options for both a vanilla-like barren landscape to a more lush (but still believably desertic) one.
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MTUI: I prefer this one to Darnified, it's far more practical but still holds the same feel as the base game.
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Interior Lightning Overhaul: Adjusts the placement, color and intensity or nearly every interior light source in the game. Subtle differences that add up for a great effect on the overall game.
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One HUD: Turns off parts of the interface that aren't currently being used, with configurable patterns. You won't know how much this improves immersion until you try it.
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A Familiar Friend - Pip-Boy 2500: This is a personal favorite of mine, it changes your Pip-Boy to a handheld version inspired by the classic games. I like it more than most other Pip-Boy mods because it's a better design than the vanilla one but still looks right at home within the game's general aesthetic. There's also
a wrist-mounted version.
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Enhanced Shaders ENB: You may not want to use ENBs if your computer can barely handle the vanilla game, but trust me, this one is the lightest New Vegas ENB around and it's also one of the best ones. The pictures don't do it justice - the effect on the look of the game is amazing, making everything more colorful and real while keeping the original artistic vision. Definitely worth a 3-4 FPS drop, less if you configure it for even better performance.
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New Vegas Restoration +
The Strip Open: Walk through the New Vegas Strip as it was intended to be. This makes the whole Strip look cleaner and nicer (but still fully recognizable) and a single worldspace, no gates. It's more performance intensive than most parts of the game but you shouldn't experience any FPS drop if you use both mods and can run the game well normally. A must-have for any player who spends a lot of time in and around the Strip.
Note: use the "NVR VMinimum" version of the first mod, as later versions add unecessary new content.
These, as well as bug-fixing mods like Mission Mojave, are essential for a great modded NV experience. You could also download more to make it a whole new game, but I like these as a base for a better, but still mostly vanilla gameplay. Enjoy.