When your talking about doing less than a full group. almost any combo works. I think casters are easier..due to being ranged ( or ranged rogues ) If you do bards, its easier to play all your bots as rogs due to the primary DPS/Heal being a channeled heal(Cadence).

Bard/rangers are great for for questiing..rift hunting or dailies. They have a rez now as well. so that helps a lot. Whatever you pick.. having a healer in the mix makes things easier as well. I have not played in of the mage class/specs yet.. i plan to level a full team of them at some point, but i hear warlock/necros are good due to pets and self heals.