The easiest one should be done because you will get less fails on hits and you will therefore get more hits resulting in more nobs. So do Excavate!!
I've not counted tbh but on average I got 1 nob every 2-4 unloads.To be honest, at the moment we will probably get freebie nobs and I think that you should do nob farming later. With the event going on its a great opportunity for you to grow.