Well, much has been made of Xan over the last few days. They're in many ways the most defensive race - their high damage often makes them the least popular to attack, though Cathaar can EMP them efficiently and many Terrans brave the high damage with their strong armour. They have been known (especially in earlier rounds) to use tactics to scare off defenders with a big, powerful fleet, rather than confronting them and risking losses with their weakly-armoured ships.
Xan Fi (+ Co)
Sentinel: Fi -> Fi
Vsharrak: Fi -> Fr
Pulsar: Fi -> De
Dagger:Fi -> Roids
Arrowhead: Co -> Co
Anti Fi (+Co):
Harpy: Fi -> Fi
Gryphon: Fr -> Co
Pegasus: De -> Fi
Spider: Fi -> Co
Beetle: Co -> Fi
Guardian Bs -> Co
Sentinal: Fi -> Fi
Arrowhead: Co-> Co
Tzen: Fr-> Fi
Lancer: De -> Co
Cutter: Co -> Fi
Cutlass: Fr -> Fi
Thief: Fr-> Co
In some ways, you can argue that you can hit every race but Xan (inits make this very very messy).
Vs Cathaar, assuming you're not going to be defended against, the most cost efficient way to attack is to send a sentinal / dagger fleet. Unfortunately, pesky galaxy mates and alliance mates send defence, so it's better to send along some Vsh and Pulsars too.
In fact, the best flak / most generally useful Fighter is the Vsharrak. The Cutlass and Tzen will be common ships, and so a large amount of Vsh will be useful in both attack and defence. Sentinals will also be useful to some extent, to make Harpys less useful and scare off a few Sentinals. Pulsars are probably the least useful of the Fi - the Bomber is probably a better option.
I'd use a fair amount of Arrowheads (Ziks will build some Cutters just to be nasty) and hit just about anyone - Terrans may not have many Pegs, so as long as you have enough Sents to disable the Harpys that's ok. Once Ziks start stealing enough ships to make them less profitable, you can switch to anyone else with a Fighter weakness - mid/end of the round, the fact it's so hard to get alliance defence is often an advantage when people get more inactive.
Xan Fr
Arguably better than the Fighter fleet. Consists of:
Ghost: Fr-> Cr
Bomber: Fr -> De
Tzen: Fr -> Fi
Sabre: Fr-> Roids
Predator: De-> Structures
(Peacekeeper: De -> Bs )
(Lancer: De -> Co )
Anti Fr:
Wyvern: Bs -> Fr
Viper: Co -> Fr
Tarantula: Cr -> Fr
Vsharrak: Fi -> Fr
Corsair: Fi -> Fr
Marauder: Cr -> Fr
As you can see, there's a lot of Fighter and Cruiser anti Fr, so Tzen and Ghosts are essentials. Bombers will probably be used more for defence.
It's obviously possible to slip Lancers and Peacekeepers in to the fleet (a Ghost/Lancer/Peacekeeper/Sabre fleet hits all Terran's anti De/Fr), but that's not the easiest combo and probably won't be that common.
Hitting Ziks and other Xans will probably be more common and easy - massing Tzens is an obvious option, as (ignoring the ETA) it's probably the best anti Fi ship in the game. Cathaars are an option, but by the end of the round you probably won't be that happy about the Viper - it's fairly efficient, and together with a few kill ships can seriously hurt your chances of landing.
Defense ship wise, Xan have some nice defence ships. The Vsharrak is the obvious choice, as anti Fr. The Lancer, Ghost, Peacekeeper and Bomber are other good ships, and some amount of Tzen to scare off other Xans is probably also necessary.
Most people will probably build more vsh than all other Fi put together, and a fairly large amount of Ghosts to keep Terran and Cath at bay, putting fewer resources into Peacekeepers. Lancers are obviously a better deterrent to Cath Co, if less useful for alliance defence AND for attack than Arrowheads (pesky Cutters!) and while some Tzen are useful, sending more Sentinals to attack looks more scary, if nothing else
Considering all but 2 of the Xan ships make alliance ETA, to some extent you can build what you want, really. You can cover all ship classes.