AT-RT is very effective at vehicular close combat fighting, and honestly, they're my favorite vehicle to use in Battlefront 2. However, the AT-TE is probably the best at various armor-to-armor combat. And the AT-AT seems most effective primarily at Anti-infantry but seems limited only to offensive objectives. The AT-AT is very tall so it can fire at targets as it climbs hills, thus softening the enemy before the following infantry even gets into view.
When it comes to battle effectiveness, the AT-TE is the best option for its versatility.