Assuming AR-15s are not what you are asking about....
AKs aren’t very hard. You will need a press and some special tools to make it easy. ( Although I have built one with a Big F’ing Hammer before)
FALs are really easy, but the barel takes some expertise to install. The rest of the FAL is easier than an AR.
UZI is easy from a 100% reciever. The parts kits are cheap for now, and the Israeli kits are cool and historical.