I have a couple of other extensions, but I like best the combination of the basic game plus The River and Inns and Cathedrals.
That said, I started thinking that a couple of tiles where missing, and decided to make them myself. Note that I always play Carcassone by taking the tiles from a canvas bag (rather than from piles as recommended in the rules). Therefore, it doesn't really matter to me whether the back of the tiles I make looks identical to that of the original set. That is, to be playable, my tiles only need to feel like the standard ones.
I used PaintShop Pro to edit scanned images of existing tiles, printed them on matt self-adhesive paper, and stuck them onto the right type of cardboard.
The first tiles I added where two each of the following ones:
Then, I decided to extend Inns and Cathedrals by adding the following two:
And finally, I extended The River by adding a river branch, (which required an additional river end) and a straight section of river:
If you want to reproduce the tiles, download the images in this articles and print them with the scale of 100%. As they have a resolution of 600dpi, they look as good as the originals!