MainConcept MPEG Encoder works best, gives the ultimate highest quality (it's a well-regarded pro program).
A decent consumer program is ConvertXToDVD.
Neither are idiot-proof, both taking a little patient learning.
Note: If you try to create DVDs from downloaded AVI files, and you don't know what you're doing, the resulting DVDs will quite honestly look like garbage. Must have patience, carefully choose the video conversion settings.