[God Emperor]

JJPHAT, what are your current tech levels? (eg shields, armour, missiles, ship size, beams/projectiles, fighters). What sort of force (fleet size) are each of the other computer players using.
One trick I used in the mid stage of a game when I was attacked by three computer players (who were each fielding fleets containing no more than four ships) was to convert my front line transports into satellite/fighter carriers (eg small transports were given two satellite and fighter bays each, med transports were given 5 of each). The conVersions only took one turn to do and meant that I could deploy all of my defensive satellites and fighters into agressive combat. This strategy served well until I was able to deploy sufficient carriers and capital ships.
In terms of who you should go after, I believe that it is a trade off between who is the weakest vs who you have the most number of shipyards near (ie logistics/ length of supply line).
Goodluck...