I've tried 'em all and hands down this has proven the best method for me, especially for the real pain-in-the-butt ones like the PIAS stickers that seem to meld organically with the plastic. This'll take it off clean with no residue nor evidence of fingernail scraping.
Peel away as much surface paper as you can, but without digging with your fingernails, which can scar the disc (for those who are OC). Spray or drip enough WD-40 to saturate the paper good and let it sit a minute. Put the disc on a flat surface and using the edge of a credit or gift card, scrape in one direction lifting the paper and glue. I usually have to repeat it to get the last bits. Wash with dishwashing liquid to remove all traces of WD-40.
Goo-Gone and other naptha-based products (lighter fluid) will work the same way. Acetone, too, but it can bleed the sticker ink into light colored discs, and you have to work faster, and it's more hazardous.