FALSE. Downturn of MW3 was easily the worst CoD map ever. Maps like Rust and Shipment had potential to be terrible maps when you played with too many people or played the wrong gametype, but they weren't consistently terrible like Downturn.
I also can't say anything about any DLC maps because I've never purchased any map packs for any CoD game and it doesn't look like the older map packs will be released for free (or even at a discounted price) at any point in time, so I doubt I'll ever really be able to review them, but I'm confident they can't be as bad as Downturn.
No. The worst map of all time is Seelow from World At War. No depth...it was just a huge boring battlefield. It was just a race to tanks, and then a 20 minute hike for the people that didn't get one. Stupid map.