Amazons were the ones that caught my eye first and I've enjoyed the most playing, or rather, the faction that speaks to me the most and goes well with my preferred playstyles. Your hero commander on a Lancer, with Veteran and Decadent for extra HP (and food decrease doesn't hit Amazons that much) is sooooo much fun and Lancer defo my fav "vehicule", almost to the point I feel that it's even superior with a leveled up hero than the tier 3 dinosaur mount, you just got so much mobility and damage, and the rest you just mod to make them invincible.
My guilty pleasure are Dvar, their core unit is just the best, even if Amazon's is amazing too. That said Dvar just adds to that aggressive melee fantasy Amazons work with too, just slower and tankier, and more melee opportunity. A melee hero commander with Juggernaut is fun to watch, just bulldozing through anything the enemy thought would save them and leaving them open for any ranged you got.
Shakarn is quickly getting up there, I've not played them much but I'm really enjoying them. As already figured, I love certain melee playstyles and Shakarn with Promethean I've found is a ton of fun with their Promethean unit being melee but still has the great AoE artillery shot. Then add the fact that Shakarn gets a pretty cool and powerful tier 3 claw weapon fairly easily that has both great melee and ranged in one, which I probably wouldn't trade for any other tier 3 melee weapon I've seen. Even then the starting melee weapon isn't bad, especially if you get the reverb AoE off. Overall though Shakarn got a fair amount of great melee units or just close-quarter, then not to mention how fun Infiltrator is at using.
I could list a lot more why I like these factions, these were just some stuff, both their theme, looks and strategic play is fun. Vanguard is perhaps another guilty pleasure just cause they're rather simple and Trooper spam with jetpacks is hilarious, and the tank is amazing. They're also a very cool generic human faction look-wise, even if they heavily rely on ranged unless with a secret tech.
Syndicate and Assembly I got a bit harder to wrap my head around, I've still enjoyed them in the campaign but I've not really learned Assembly completely to get used to how they work, and Syndicate can require more thought into it and I need to play them more. Kir'ko is similar, I've only played the campaign missions with them and not really dug into them completely. Both them and Assembly I feel I should like for certain melee options, but they don't click with me yet. I probably just need to play them in normal scenario, make a commander and pick secret tech I feel I'll enjoy with them.
Still, there's no faction I downright dislike (which is amazing) and probably won't play again, each one has its moments all around, with the same for secret techs. It's both fun playing them properly with secret tech that fits, or just trying more unorthodox ones with a more mixed playstyle. Same for when making a commander and making one that really clicks with the "roleplay theme" you might be going for.