If you are working on specifically upgrading a shrieker rush I would say graboid for sure. It generally gets played earlier and even though it takes a turn to become a shrieker, your first out shriekers are still usually evolved graboids. Shrieker rush is also a deck that generally uses a lot of pillars, so upgrading one pillar has less of an effect than in a deck with fewer pillars. Upgrading all your pillars makes you one turn faster so upgrading one pillar of 16 makes you 1/16th of a turn faster, while a deck that only needs 10 pillars is made 1/10th of a turn faster by upgrading a pillar on average. Also graboid is used in more decks and it is good to have it upgraded if you want to run a speedbow or other deck that uses graboids.