My concepts are built around an amalgamation of the maximum of two purposes per ship type (at least with military vessels). The usual elements comprise firepower, defence, speed, range and sensors.
What I still want to try is to build an armed freighter out of a Large or Huge military hull (this way it could travel safely and would usualy survive pirat, etc attacks

). So, in order of appearence:
------------------------------------
-1: Tiny Chassis
1a) Light Fighter (built for range & speed as
space fighter)
1b) Medium Fighter (built for firepower & speed as
system interceptor)
1c) Heavy Fighter (built for increased firepower as
planetary defender)
1d) Scout (built for sensors and speed as
basic scout)
-2: Small Chassis
2a) Corvette (built for improved firepower as
patrol gunboat)
2b) Corvette (built for long range and speed and sensor as
improved scout)
-3: Medium Chassis
3a) Frigate (built for long range, (average) firepower and sensors as
the Fleet's main support vessel)
3b) Destroyer (built for firepower and defence balance as
the fleet's backbone)*
-4: Large Chassis
4a) Light Cruiser (built for firepower and speed as
fast attack cruiser)
4b) Heavy Cruiser (built for firepower and (only marginary) defence as
gun cruiser)
-5: Huge Chassis
5a) Battlechruiser (built for firepower and speed as
"cruiser killer"
5b) Battleship (built for firepower and defence balance as
Vanguard Battleship)
------------------
-6: Cargo Chassis
6a) Cargo Freigther (built for speed)
6b) Constructor (built for speed)
6c) Colony Ship (built for speed)**
6d) Armed Troop Transport (built for speed; and for increased troop capacity- between 2000 and 5000 troops)
------------------------------------
* aiming at adding an extra hull size between "medium" and "large" for the destroyer size, to go with the intermediary technology between those two classes
** while I have designed one I have never actualy gotten to build one since 90% of all planets are usually taken in the first 50 turns