The biggest item on Brad Treliving's off-season to-do list is to add a starting goaltender.The Calgary Flames appear ready to move on from Brian Elliott and Chad Johnson is definitely not a guy you'd want to hand the reigns to on a full-time basis.There is no shortage of options available on the market -- several teams will be looking to move goaltenders in advance of the expansion draft rather than losing them for nothing -- but I think Treliving's best course of action may be to sign free agent Steve Mason, assuming he makes it to July 1.If the Flames trade for Marc-Andre Fleury or Philipp Grubauer, for example, it will cost them assets. If the Flames decide to re-sign Elliott it will cost them an asset (they'd owe St. Louis a 3rd round pick in 2018).The Flames are already without a 2nd and a 3rd this year so it's probably not a good idea to continue handing out picks, even though the team has a very good farm system.That's why signing Mason would make so much sense. The Flames could retain all their assets while quite possibly getting the best goaltender available this summer.Mason is coming off an up and down season but his overall body of work with the Flyers is quite impressive.From 2013-17, Mason posted a .931 save percentage at 5v5, which is good for 4th in the NHL. He did that while carrying a starter's workload (he averaged 56 games per season).The only goaltenders who posted better numbers at 5v5 over that four-year span: Carey Price, Braden Holtby, and the underrated Scott Darling, who did so over a much smaller sample size.Mason has quietly developed into a very good goaltender and his asking price would likely be quite reasonable given a) he isn't a big name; b) he isn't coming off his best season and; c) there simply aren't many starting jobs available.Treliving has a lot of options to choose from but Mason may very well be the best of the bunch.