Monitors would be the largest way of doing things, but Traveller's BattleRiders benefit from some of its warship construction idiosyncrasies, such that a 15,000-tonner can carry the same heaviest-gun that a 200,000-ton battleship can carry, and each ship is only allowed one of them.
While at any time some big assault dropships are capable combat vehicles, they have a lot of flaws against true WarShips (or even monitors). Less once sub-capital weapons come along, but given how incredibly light capital-grade weapons are, I find it silly that dropships aren't allowed to use anything but capital-grade missiles. If droppers were allowed to use capital-grade energy weapons, they'd be a much greater threat to WarShips, more the way Traveller's battleriders are able to threaten any large warship.
On the other hand, large assault dropships can be rapidly transported by JumpShip; as I recall, although monitors up to 300 kton can be jumped, it's not as simple as hooking them up to a docking collar on a JumpShip, which limits their flexibility considerably, and brings a 20,000 to 100,000 dropship back to being the closest thing standard BT rules can provide to a Traveller BattleTender & BattleRiders.
