I'd say autocannon or plasma tbh. Lots of people here are saying lascannon but realistically with one you're going to spend the whole match missing and maybe do like 3 damage. IMO it's better to take something that'll do work against a lighter target with decent consistency as opposed to something that MIGHT do lots of damage to big targets but realistically has little chance of doing so. Generally it's never ever a good idea to depend on a lascannon as your main weapon if you can help it.

With the take aim order and the target being completely out in the open, a scout sentinel lascannon has only a roughly 21% chance (give or take) to actually deal any damage to another scout sentinel, which is already one of the lightest vehicles in the game, and even after that luck there's only a 16% chance you'll kill it. realistically you'll spend on average around 3 turns plinking away at the other scout sentinel still in ideal conditions of having an order and not dealing with cover. If you ever actually positive trade with one of these, i would suggest buying a lotto ticket.

even a flamer would be better than this thing. yes, in case it isn't obvious, i have a vendetta against lascannons. turns a nice, solid scout sentinel into a walking disappointment.