Shatter is the frontline option that pops the most frequently(if its available you can just use the frontline roulette - daily bonus isnt bad). Astragalos seems popular - still has that new car smell. And the feast gets plenty of participants too. The other two maps(secure and seize) are certainly more rare. Ive gotten both from the frontline roulette, but they seem to happen at less then 1/10th the frequency of the shatter matches.

If you want to get the fastest ques, make sure your language options match the data center you play on. Enable the Frontline Freelancer option and the party in progress one too. And if its an option, playing at close to your servers prime time also helps. If you are determined to get a specific rare map then que directly and wait(all the speed things up tips still apply). You will eventually be at the front of the que, and eventually get matched up with enough participants to get at least the small version of the fight going.