It really depends on
1) How you like to play
2) What things you find annoying
You can find a list of modifications for the main game of TW1 at http://en.thewitcher.com/forum/index.php?/topic/19890-list-of-mods-for-tw1-updated-24-dec-2012/
You can find a list of fan-made adventures for TW1 at http://en.thewitcher.com/forum/index.php?/topic/19893-list-of-fan-made-adventures-for-tw1-updated-24-december-2012/ There's also one fan's review of the adventures
here.
Since it's Valentine's Day, I'll just mention that there's a very short
Valentine's-themed adventure.
The most popular mod for TW1 is
Flash's Full Combat Rebalance, usually abbreviated to FCR. It makes the game significantly harder and also offers the player a lot of options, such as having the flaming sword on Hard.
The most popular mod for people who want to cheat or to make the game easier is the
Genie Wish Mod, which allows the player to wish for weapons, armor, potions, sword oils, talent points, or gold.
I've made a bunch of mods that fix bugs or eliminate annoyances; you can find them at the
Witcher Nexus.
There are also mods that change the textures.
Really, it all depends on your tastes. There aren't so many mods for TW1 that you can't just scroll through the list and see what grabs your fancy.
There's also a
Saved Game Editor.
