Erhmergerd, just spam paris like duhhhh ebeejeebeejeebobedyboopedyaaay.
But on a more serious note, largely depends on what stage of the game.
Covenant Early: Couple of ORS cruisers supported by around 4 SDV's (corvette thingys),
Mid: Moving into the mid game (by that I mean once you've taken the first 3-4 planets) move onto some DDS and DOS capital ships with supporting SDV's and CPV's and start foraying toward your enemy (hopefully your scouts will have found them by this stage), keep on expanding using your ORS fleets whilst your DDS and DOS fleet keeps pressure on your enemy to try and stop them expanding. If you're fighting a covenant, try and thing their expansionary fleet and kill it, if it's UNSC, just keep pressure on, destroy constructors using fighters/bombers to prevent them building SMAC's, nuke capital ships using DOS', also kill frigate factories. So in list of priority for a gravity well that is being developed or is developed: Frigate Yards - Constructors - SMAC's - Fleet (if there is one), whilst keeping your ships away from any built SMAC's.
By the late game you want to be building CCS's and CPV's on a 3-1 ratio, along with capital ships. Don't build more than 4 DOS and DDS's, since spam isn't really the most efficient thing to do with these caps, rather try and get a few tanky caps like CAS and OSS.
UNSC: don't play them
Early: 2 Orions at the most, and Marathons. If you want to build some Paris' as fodder, fine, but don't build more than 10.
By your fourth planet, you will want to have built at least an additional 4 marathons (So your fleet composition should look like 1 Orion, 2 Marathons, 6-10 Paris, Maybe 1-4 Stalwarts). Always build SMAC's around your infrastructure once you've placed it to fight off any enemy raids, or at least to have a bit of preparation if they attack when you're unprepared
Mid: You should have a few Epoch's and Artemis' as their fighter/bomber units + power MAC's complement eachother well, try to build some frigate yards and capital yards close to the front, if not, make sure to have build Anchor stations (for both Trade and ship construction). Halcyons are very useful tanky fodder, and I recommend replacing a majority of your Paris' by the mid-game with these, as they will last infinitely longer, you can also build a few carriers (not too many as they cost a lot in fleet supply) to complement your Epoch's available strikecraft. You ought to have built a few ONI Sloops (the cap ship varient, can't recall its name), in order to cloak and foray into your enemy territory, (especially if covenant) to target and destroy and weapon tech labs, as you want to avoid your enemy having the ability to call in high-level patrol fleets, as well as high-level cap ships and cruisers.
By the late game, you should have kept up your number of capital ships, ranging from 1-6 Artemis, 1-4 Epoch, 2 at the most Orions, 4 at the most Punic's, and enough Marathons to make Lord Hood green with envy.
Try and keep up the pressure on your enemy's infrastructure (It's not easy), and make sure to upgrade planet infrastructure and build SMAC's (with some saber platforms around them to offer AA defense)
I just gave away all my secrets :(
and sorry for any grammatical errors, I wrote this very quickly and haven't proof read
Don't touch Paris. Spam Halcyons and high-damage capitals and build clusters of ODPs and hangars in orbit of your borderline worlds. Have some Artemis-class capital ships near every ODP cluster in order to deal with enemy DOS-class long range battleships.
The rest you can figure out yourself.
UNSC: Build Orion Carriers and a couple Marathons to escort alongside no more than 10-20 Paris per fleet as a basic composition.
Research the Destroyers for Anti-Ship and Stalwarts for Anti-Air for later fleets to replace the Paris.
Use the Orions to colonise planets whilst the escorts attack the pirates and whatever else is out there. Having a large number of Stalwarts can allow you to build carriers that solely have bombers, as they will help finish off the ships hit by MAC rounds and draw the attention of the enemy fleet and accompanying fighters. This will also expose the enemy bombers making a run for the fleet allowing the AAA to take them out quickly.
Keep the number of Sloops roaming up, send them out in pairs in case one is destroyed. You can use the Heavy Prowlers, but they need constant attention, as they don't have the "Explore" ability. Leaving mines in each system allow you to monitor the system.
Only build the Infinity if you have an enormous excess of resources. Smaller ships can get the job done.
That and you need a certain Spartan's Luck if you're fighting the Covenant.