I say cantrips as rouge gains 1d6 every 2 levels so at level 10 you would be doing an extra 5d6 + the extra damage spells do at later levels. So a scorching ray cast at level 3 would do potentially 20d6 sneak attack + 8d6 fire + 8d6 necrotic with hex cast so 32d6 total which is 9th level spell damage, would kill almost any monster on 4 hits in the same turn.
A tenth level arcane trickster would do levelled cantrip damage + 5d6 on sneak attack so a little more than a crossbow or something.
