First Calamity playthrough - which class should I pick?

whichever class you go with, install boss checklist alongside it. calamity adds a lot of bosses, and the checklist shows the order they're meant to be fought in and how to summon each one. saves you alt-tabbing to the wiki every half hour.

Rogue is worth a look since it's the class Calamity adds itself. Throwing weapons plus stealth strikes, and it feels nothing like vanilla. Otherwise ranger or mage. Both hold up through the whole mod without a real weak patch. Summoner gets a lot more love in Calamity than in vanilla but the early game is slow, so I'd leave it for a second run.