The default cars eventually got boring for me once I beat the game. I made a list of the top five of my favorite downloaded car modifications for Grand Theft Auto: San Andreas. Installing these mods requires the San Andreas Mod Installer (SAMI) program which can be found in numerous locations online, such as here.
All non-credited pictures were taken by me.
The Chrysler 300c DUB Edition

This is the fastest car in my GTA: San Andreas game. It has super fast acceleration and goes to extremely high top speeds. This makes the car slightly hard to control when turning corners but it isn't anything unmanageable. The downloaded mod by default replaces the Virgo, so you can expect to find it in the poorer sections of town. The 2005 Ford Mustang GT

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A very good model of the Mustang GT. Unlike some of the other mods I have downloaded, this one is made so that the rear lights go red when you press the brakes. By default it replaces the Bullet, so it also has two color sets that are changeable in the mod shop. One thing I found about the car, though, is that it is very low to the ground. So low, in fact, that the curb can damage the car or, depending on your speeds, send you flying into the air. The Hummer H2 Stretch Limo

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I love this car. Instead of the boring default limo that came with the game which, no matter what color you paint it, has those dirty marks, this Hummer comes shiny and clean. I didn't see any noticeable different in handling or interaction, but it's much more fun to drive around in this. Obviously, this mod replaces the Limo by default. The Hummer H2

Just like the Limo in Number 3, the Hummer here makes it more fun to drive around. It replaces the Landstalker by default so it tends to be found in the desert and country side. Again, I didn't find much difference in handling so I'm not sure if that's a bad thing or a good thing. I would recommend downloading this since it looks awesome in black. The Cadillac CTS-V


Finally, my favorite GTA San Andreas car mod - The Cadillac CTS. I'm not sure what year the car is but either way it's beautiful. It's a fast car that takes very wide turns at certain speeds. It looks amazing in black. It would look as great in white too, but then it becomes too shiny and hurts your eyes a bit.