In my current game I'm playing the Drengin on a gigantic galaxy, with abundant planets. Difficulty is set to challenging with randomized intelligence turned on.
The Altarians were at war with the Yor, and I noticed they had a massive number of undefended planets on my borders. So I spent some turns churning out transports with 31 speed, lined them up on the borders, and then struck: I captured 21 planets in a single turn. The next turn I mopped up and captured 4 more planets.
It far from crippled the Altarians, but it's certainly the largest surprise attack I've pulled off.