For most North American sports fans, the success of one's team is generally defined by making the playoffs and winning the Championship of their respective league. Now, I'm sure the North American soccer fan probably views it this way as well. In MLS, that equates to MLS Cup and the Supporters Shield. I have no issue with those who want to measure their team's success using those barometers.
Give that soccer doesn't conform to the North American standard of competition (league play only etc), I often wonder, given the uneven playing field of MLS, that are the 2 major trophies really that attainable to all? Yes, I'm fully aware that Portland won the 2015 MLS Cup, but given their 2016 form, one could say that they caught lightning in a bottle (Saad Abdul-Salaam's double post) and ran with it.
Given how that a lot of the benefits that come with the Supporters Shield and MLS Cup are US Soccer spots to the CCL, a case could be made that they don't really hold the same value to a Canadian club than to their American counterparts.
One thing that I love about soccer is the Cascadia rivalry that has existed between Vancouver, Seattle and Portland since the 70s and has culminated in the Cascadia Cup. Unlike the other "rivalry cups" out there, MLS does not solely own the trademark, thanks to the Emerald City Supporters, Timbers Army and Vancouver Southsiders who banded together to fight this trademark. As such, they were successful in keeping some control of the Cup in the hands of the supporters. This is one of the primary barometers I use in evaluating the season.
Another major barometer I use: the Voyageurs Cup. It's the Caps only way into the CCL, and I don't see that changing any time soon. The victory in the 2015 Voyageurs has seen the Caps survive the group stage of the 2016-2017 CCL and advance to the QF Stage against fellow MLS side, New York Red Bulls. I won't re-hash what happened during the 2016 Voyageurs Cup, but needless to say, it won't be an issue come 2017.
To sum it all up, I put Cascadia Cup and Voyageurs Cup at the top of the priority list, followed by MLS Cup and Supporters Shield.