Well I'd say Big Gundown and At the End of the Rainbow. Then perhaps Django the Last Killer, Train for Durango and Killer Kid. I haven't seen half of the films though (the latest ones except Django the Last Killer) even though I own the discs.
Koch's Big Gundown disc is out of print though so you may have to pay extra for it (cheapest marketplace item at Amazon.de is 20euros...). And May God Forgive... But I Won't hasn't been released.
EDIT: Good choices Stanton.