Iri/Shim tend to look fantastic on Tundras, no matter what color combination they have. Even Obsidian Iri/Shim tend to have a nicer look than normal on Tundras. [At least if you aren't a fan of Obsidian Iri/Shim... I am not particularly in love of Obsidian in general as it is. X3] My favorite Primary gene on them, however, has got to be Tiger.

They are so fuzzy, the tiger stripes just fit!
For Secondary, I tend to prefer Freckle. Tundras really make Freckle shine, as it isn't just their wings that get spotted, but their entire manes and mohawks too, like these two show.

Please note though, that I am hopelessly biased when it comes to Tundras, and for the most part I tend to love all genes on them. Due to their inverted color scheme when compared to most breeds, you just have to be a bit more careful when planning which genes to give, but for the most part, I think you can't go wrong with any when it comes to Tundras in general~ I suggest looking around at certain gene combinations in colors you like to see what strikes your fancy most with them. ^^