With the utmost genuine respect to Jamie Moyer, when it comes to the best pitcher in Mariners history it's a straight battle between Félix Hernández and Randy Johnson. It really does seem like a case of 1A and 1B, with plenty of reasons to pick either "King Félix" or "Big Unit" and feel good about your choice.
In the end, the edge arguably goes to Hernández due in part to just having the longer career in Seattle, contributing to him holding plenty of team records. Among other things, he's ranked first with a 49.9 bWAR, 169 wins and 2,524 strikeouts, as well as tied-first with a 3.