LifeMiles will probably provide the most redemption value for you as you will likely be flying mostly Avianca being located in South America. If you fly Copa a lot, then you might go with MileagePlus, although that program has been recently devalued and could go revenue based soon, which will make it harder to get much out of it as a low spender flyer. LifeMiles will probably be easiest for you to get status on, however they require some flying on Avianca/TACA. Aegean has the lowest qualification thresholds of the entire alliance but many fares earn only a fraction of what they would earn on other programs and some discount economy fares earn nothing at all.