Egyptians have pretty good synergy with Genesis Heroes: Joseph can search for (or protect, depending on the version) Storehouse, which can be used by The Dreaming Pharaoh to draw; Pharaoh's Baker can count Genesis Heroes for his X value; Seven Years of Plenty (and to a lesser extent, Famine, depending on the strategy) goes well with both Gen Heroes and Egyptians. Also, since many of the best Egyptians are Genesis, if they get converted (by you or someone else) they don't egatively affect the abilities of Zebulun and Benjamin.
I personally haven't tried an antediluvian-themed defense, but there is also some synergy there, but that would be more aimed at Flood Survivors, so it depends on the offense you have.
Philistines and Romans have historically been good pairings with a Genesis deck, Philistines because they are more character focused than Enhancement focused, so cards like Eve and Noah help the defense out well; Romans because they go well with the theme of hand-thinning that is popular among many good Genesis cards (the 2008 National Tournament winning deck was a Genesis/Romans hand control deck).
There are tons of other options of course, but I'd say those are the top 4.
