I'd use only red heterozygous for agouti (Ay/A), agouti, or cinnamon.
Chocolate or black tan are the next best options. but then you will have animals who are A/a or A/at instead of A/A for a while.
Make sure that whatever you're using doesn't carry any blue, C-locus recessives, pink eyes, spots, non-standard coat-types, etc. Those things can take years to remove from a line once you introduce them.
Are you trying to improve color or type or both? If it's just color you're after, red might be the best bet. If it's both or just type, red isn't going to be the best--I'd go for cinnamon if possible.