Light Armor is the way to go usually. Any structural block (Armor, Interiors, Scaffolding, Beams) will be evaporated by railguns or torpedoes anyway, so there's little reason to
Heavy armor can be used in critical places. It is marginally better at tanking PDC fire, but the trade-off is its greatly increased weight. It is rare for meta ships to have more than a handful of blocks of HA, and most have none.
Carbon Silicate armor can be used to up-armor your ship, it is sturdier than LA while being only a little heavier, but it is hard impossible to farm at the moment.
Most combat ships should be able to do 10 g or roughly 100 m/s² of acceleration to be competitive. Going big is the best way to accomplish this - use large numbers of 5x5 or 7x7 drives to get about 400-500 km of drive signature under full thrust.
Most ships don't need more than 10 RCS per face. Lower amounts save on fuel consumption and PCU.
Railguns should always be able to shoot both forwards and backwards with minimal obstruction from the hull or other weapons.
Most ships can power no more than 3 railguns OR 6 coils before making significant tradeoffs in other areas.
The coilguns' model extends a block above what it appears, so to be able fire over a coilgun in front of it there must be two blocks of elevation difference between them.
The majority of a ships PDCs should be able to fire forwards AND backwards, with an emphasis on backwards. The farthest projecting shapes on a hull should be reserved for PDC mounts.
There is no hard answer for "how many PDC's should I have", but top level ships with lots of expensive parts have 30-50 these days. The cheaper the rails/torpload, the less you should spend defending it.
Torp launchers should be oriented for vertical launch whenever possible, or horizontal if that isn't possible. Forward launch may be safer than it used to be, but its still possible to nuke yourself.
All ships should have at least one backup spawn point and cockpit.
Multiple conveyor lines should run the length of the ship, and ammo/fuel should be stored in multiple places in case lines are severed.
Don't use large h2 tanks - many small tanks are both safer and more redundant.
Most combat ships need 6+ large reactors. Plan your size accordingly.
Use a large number of batteries - ideally Tycho batteries - set to discharge in combat or recharge out of combat to store power and draw railgun fire away from your reactors.
