Unfortunately none are really uncommon. In order I'd check: valve covers and cam seals (seriously, just replace them and don't forget the FTV at the #1 cam caps), then the oil pump drive seal, then the lower rear timing plate.
Cam seals and cover leaks will be fairly evident as they're the only two which will leak a decent amount of oil behind the upper rear timing belt cover (the black plate).
If the head hasn't been removed it's VERY uncommon for oil to leak from between the head and the lower rear timing plate... unless you're VERY unlucky (raises hand) I put too little on or it didn't cure or something, it blew out a few months after doing the HG on my personal car. If the head was pulled and the RTV was not used though it WILL leak.
Did you replace the valve cover seals? Did you over torque them (only 22 INCH lbs, NOT ft lbs!!!)?
As I said before, I'd pull the upper plate, clean out behind it thoroughly, replace the cam seals and valve cover seals, reassembly, clean the rest and run it... use some dye and as long as you're paying attention you should see where the leak is quite readily if you didn't just fix it.
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