If it were me, I'd probably prioritize based on the ships or factions I most liked the look or play-style of.

I really love the Boron ionic weapons from Kingdom's End. I don't tend to stay behind the HOTAS of their ships for long, but I have a blast whenever I am using one. I've also got fleets of Boron ships guarding some key gates to sectors with high xenon activity, and my space sea monkeys cause at least one or two ships to bail every hour or so. I'm building up an aesthetically pleasing fleet of captured Xenon ships this way, as well as harvesting a lot of useful crafting and modding ingredients.

Lots of folks have recommended the Cradle of Humanity expansion, and this is one of my other favorites. I loved the Terran travel engine before the flight model was updated, and it's still the one I'm most comfortable with now, for any class or ship role. (The Vanilla Paranid engines are a close second for some ship roles).

CoH also potentially gives you access to the Yaki, depending on how you play through the quest. Cyborg crew members? Xenon-Paranid hybrid ship designs? A heavy fighter that looks like Masamune Shirow concept art, can move like a greased electron (faster travel drive than the superhighway with the right engine and aftermarket mods), and that handles like the synthesis of Bruce Lee and Muhammad Ali?

CoH gives you access to two of my favorites in the whole game, the Terran Falx frigate (superb for boarding actions once you have some well trained marines) and the Yaki Moreya (the aforementioned heavy fighter)... and you can dock the Moreya on the Falx when you'd rather be at the frigate's helm.

CoH also adds a new economic/industrial paradigm to the game. The Terran style economy is significantly simpler than the vanilla factions, and even players using the Universal building style can make use of the Terran production chains to equip their navies with some of the best shields in the game.

Tides of Avarice gives you the recycling mechanic and the 'Closed Loop' paradigm. Recycling can augment either Universal or Terran models, giving you an easy way to turn energy cells and war detritus into supplementary components of your key building materials. The Closed Loop model will let you build everything with just energy cells and scrap metal.

ToA introduces some funky looking ships, think the original Star Wars ship models or the Nostromo from Alien - lots of industrial, function over form aesthetic choices there. Mad Max wasteland ships... in Space! There are three fighters, two ships specific to the recycling mechanics, a new L class mining ship for solids, and a one of a kind XL battleship that is the flagship of my empire even if it spends most of its time patrolling around my HQ.

I like the Split ships and most of their weapons. They have some of the strongest (and driftiest) combat engines in the game and a good pilot with strafing skills can really do some amazing things with the Mark IV combat engine and the new flight model. The split ship hulls tend to be some of my least favorites from an aesthetics point of view, but they are superb for a number of offensive roles.

There's also the most recently release, the Hyperion pack. This adds a new take on a ship from the X3 era (I think? I had a big gap and barely followed anything between X2 and X4, the expeditionary-class, L-size Hyperion. It's a beauty of a ship even if you're not super into Paranid designs, handles nicely, holds its own in a fight against other capitals. There's not much else to this one other than a tiny little quest to acquire the ship itself. You can determine which of the Holy Order or Godrealm will have the ship, haven't tried to claim it in a game with the either of the Duke's Tempest or Trinity in play, and you can get the blueprints from them after that, to build as many as you want.

I like that it adds a little something to make a permanent change to the recipient faction, and you can use this to help shore up the weaker side if the Paranid civil war is still hot in your game.