Aerolineas, via Buenos Aires, usually pretty cheap, and I have flown them quite a few times. Not a great airline though, even in business, but especially in economy. Lan is a lot better, and much more direct, but usually a lot more expensive.
Sometime BA have a RTW special (try bestflights.com.au - I have seen it there before). I think it goes something like SYD-SIN-LHR-EZE-SAN-SYD using QF & BA codeshares. Probably works out more expensive than LAN direct though.