i just finished reading the photek v SD thread (in it's entirety, and i'm now exhausted btw), then went back to the main page to see this thread
I agree with Phokus though - I've always regarded Reinforced as 'the dad' and Metalheadz as 'the son (that might be able to give the dad a kicking one day)'.
Both amazing, but for me, Headz obviously owes it's existence to the big R
edit: this doesn't even answer the question, does it? Gotta be Reinforced though, hearing Underground Software and 4-Hero were half of what got me into this shit for life. and then there's Enforcers..... (not denying buying Terminator EP wasn't a seminal moment though )