Space marines. Play a bit like better Imperials. They don't field traditional cruisers, only light cruisers which are cheap and flexible, but not especially exciting. Their escorts are basically better imperial ones but slightly more expensive. The big advantage here are the Battle barges, which they can usually field more of than other sides can field battleships. Battle barges have good armour, good shields and a planetary bombardment gun they can use against ships.
Bombardment cannons are great, yes, but short ranged. There's not much long range firepower in the fleet at all. Iin my opinion the main asset of Space Marines are boarding and hit and run. They really excel at that! Firepower alone probably won't do it for Marines. Boarding torps, Thunderhawks and teleport attacks seems to be where it's at (judging from reading the list and my one game I had against them a couple of years ago...)
As I explain to friends - Space Marines mostly shoot Space Marines at people. Boarding torpedoes on everything is incredibly useful, and boarding actions - with the Space Marine bonus - are lovely. Bombardment cannons work best coming out of a full cruiser squadron (6+ front armour, you say? Nah, I'll hit on 4s instead. Critical damage on a 6, you say? Meh, I prefer 4s again).
Nobody's really talked much about Tyranids, which makes me sad. They're a great fleet for fun. The instinctive behavior can result in some screaming at your own fleet, but it's still entertaining. "Not the planet! No! Turn around! It's not lunchtime yet! Kill enemies,
then eat! You little morons!" "You're...boarding!?! It's a battleship! It's twenty times your size! I don't
care if you're hungry, it's still way too much for you to take on alone!" And perhaps the most aggravating one: "He's right next to you! He's right there! How did you not notice him!?!"
Tyranids have a flexible fleet in terms of what you build. Like Dark Eldar, limited ships types, lots of options. They're slow but dangerous - spores give most of your ships decent shields/turrets, and they work on pretty much everything. Hive ships are the core of the fleet, and while they can sport impressive firepower, they can also deliver
hordes of assault boats, as Tyranids can pump out twice the craft of any other fleet
and anytime it reverts to instinctive behavior, it will reload ordnance (unless there's something more interesting to do). And while the slow speed limits your range threat, it also means that your craft are so closely packed that enemy escorts can't get near you. Cruisers are light, and best used for boarding (Tyranids put everyone else to shame when it comes to boarding actions). The three varieties of escort are all useful - the slowest ones sport very good weapons batteries, while the two faster versions can both be equipped with feeder tendrils, enabling them to tear other escort squadrons apart and wreak havoc on larger ships without endangering themselves too much. Kraken, the escorts with a permanent saving throw (very rare in BFG), can also sport the heaviest weapons batteries of any escort in the game, if you want to make a mess out of Eldar.
Tyranids are disgusting at low-point games. While most fleets have to take cruisers in order to take bigger ships, and pretty much everyone else is limited in how many Battleships they can take, Tyranids build their fleet by
starting with the Battleship and going from there. The result is having an absurdly hard-to-damage ship in a game where your opponent lacks enough firepower to really threaten it.
All that said, bugs aren't hard to play - especially since once you start failing Leadership checks, Instinctive Behavior largely runs your fleet for you anyway. They have very limited weapons options and only 5 total ship types. And with your fleet trucking toward the enemy at 15cm a turn, you'll have plenty of time to plan out your strategy before you can begin shooting back at them.
