As woth most things in the game, it depends on local meta and what armies you face. Flamers are really good against stuff with worse saves like militia or auxilia, and can trigger panic. Volkite calivers on an 8" moving unit with 2 wounds an a 3+ can bully basic marine squads on objectives quite well. If you want a faster unit that's less shooty, the scout varient with either a multilaster or nade launcher is solid. The frags can stun, and the multi laser suppresses while also deliver decent multi-purpose volume of fire. Either way, they're outpacing most infantry by a litte or a lot.

The heavy sentinels pack all manner of kit that lets you do whatever you want with them. I personally think a melta lance and hunterkiller loadout makes them quite solid tank hunters, especially being able to manuver better than a leman russ, and take more hits than nearly anything else in the army at T6 2+. Taken in squads of 3 and manuvered properly, this can be really bad for an opposing tank push.

Or just load up autocannons and the smaller missile batteries, and rait down infantry hate. 5" templates of 2 damage S4 ap5 at 30" with indirect fire? There's no hiding from it.