There isn't one quite frankly. Lock on is bug ridden, even with latest patches and addons, and you'll only find the original in the high street (if you're lucky). It runs like, well, poo too. Fighting with the AI is extremely dissapointing, but that is pretty much what lets down all games for me. You're only other alternative is F4:Allied Force, which is a souped up 10 year old game that still looks like crap, and is, in all honesty, a million times harder than Lock On. I mean seriously, even if you were under the impression you were a aviation fanatic, unless you have 6 months non stop to invest it'll just gather dust. If you enjoy a game for it's technical ability and undertaking however, nothing really comes close.
Lock on is good for a blast, but with zero depth (even if it does model relatively complicated avionics and flight parameters). F4AF is an extraordinary game, but I can almost guarantee without knowing you in the slighest that you won't enjoy it, at all.
To err is human. To really foul things up ... you need a computer.