For SkyTeam, I picked KLM Flying Blue because I end up on a KLM flight once a year while I was on a Delta flight on Good Friday for the first time in a decade (and the previous flight was once my only time on DL). That choice was originally made to evade a creeping rental car tax for FF points (first to US carriers, then added Canadian ones, now all) but Flying Blue is also practical for point collecting opportunities in Europe - Accor Hotels and some boutique hotels, and once a pharmacy in Amsterdam, where I could not use United, American nor Air Canada (my Star Alliance backup for trips to Canada, and Delta Hotels).