I can't settle on a first faction to start with. Each one has great models, art, and cool toys. How did you decide which one to start with? Is there a personality quiz somewhere I can take?
In that case I will shamelessly advocate Yu Jing. They have gorgeous sculpts, can play various list archetypes trough their sectorials while still remaining viable in Vanilla and are, of course, the only real superpower in the human sphere.
I like their orange/yellow color. I love power armor. How's their set from Red Veil stack up given that it's a couple years old now?
The sculpts are up to date in terms of quality and scale. As far as unit choice goes, the starter gives a nice mix of units for a Vanilla Yu iJing list. If you consider getting this, try finding one with the Ninja KHD included (it used to be exclusive to the Red Veil set, but you can still find the starter + ninja on eBay). The Ninja is an amazing sculpt and a very useful unit for Vanilla YJ and Imperial Service. If you want to shoot for a particular sectorial or list style from the get-go, you could skip the starter and buy miniatures more selectively. I'd suggest digging into Infinity Army and figuring out what kind of list looks most intriguing to you.
This little breakdown of the different factions by @TheDiceAbide might help you: https://www.thediceabide.com/blog/2018/8/3/infinity-which-faction-is-for-you It's not entirely up to date, the latest NA2 sectorials (spiral corps, dashat company, foreign company) and O-12 are missing, but it's still a good read!
Abuse the ability to open new tabs in your browser. Go to the CB Webstore, look at one faction, and open each unit that you really like in a new tab. At the end of the list, count how many tabs you have open. Then go to the next faction and do the same thing. Choose the faction with the highest number of tabs of cool models that you like.
While still very playable, some sectorials have been retired from production. This might mean you would be limited to proxy models, or scrounging a bit to find discontinued pieces. I would probably recommend going with an army that is still in production for simplicity of list building. Also note that the newest armies may still be missing a few models, so while they are probably on the way you might have to proxy there, too. If you are new to the hobby entirely, you might consider a lower unit count army, too. This can help keep you focused while getting things painted up for play, and then is flexible to expand out.
To expand on the point made by @Vanderbane the forces that aren't currently in production are the sectorials of Merovingia (Ariadna), Acontecimento (PanO), and Qapu Khalqi (Haqqislam). Any Tohaa models not in the Spiral Corps are also out of production. They will remain playable in perpetuity and will likely return in the future but probably best avoided for new players at the moment. Likewise O-12 and the Shasvaasti sectorial of the Combined Army only just came out with new model ranges, so they're fairly small at the moment, and will get new releases for the next 6-12 months. Anything else is pretty golden. If you're learning the game from scratch then I usually recommend one of the forces that is in a battle box (Operation: Icestorm/Red Veil/Coldfront/Wildfire) as they have great introductory tutorials, and the Beyond Box for each set has a great complementary PDF that bridges the gap between the tutorials and full 300 points games. If you're learning as part of an existing group grab a force you like the look of and are willing to put in the time to paint, some people prefer to focus on a sectorial as it limits the available choices and stops you getting overwhelmed, others like to jump right into Vanilla so the grey all the funky toys. As a very brief, not altogether accurate guide to the factions: Panoceania: Shooty bois Yu Jing: Power Armour bois Ariadna: Camo boys with Werewolves Haqqislam: Generalist bois with super Doctors Nomads: Hacky bois Combined Army: Evil alien bois who break the rules Aleph: Android bois O-12: Law bois (very new, not actually in shops quite yet) Japanese Secessionist Army: Stabby bois Spiral Corps: Not as evil alien bois Whilst each faction has it's own definite flavour most can do hacking, Heavy Infantry, TAGs, camo, close combat, etc to some degree (except for Ariadna who trade TAGs and hacking for werewolves). If you pick a Vanilla faction you'll rarely get pigeonholed into one playstyle.
Well you must have a selection of factions on a short list. Check out human sphere entries for those and see what kind of people are attracted to that faction and what strategies are.
One more comparison list to further muddy the waters for you: https://1d4chan.org/wiki/Infinity_(wargame)
If you can’t decide between two faction’s models, see if some of the Merc factions like Dashat or Foreign company allows you to field the minis you like most in one faction.
Playstyle. I wanted hacking superiority... and fell into Aleph's "trap" (at the time, 4 years ago). Then Nomads. Nowadays the distinction is muddier, it's not as easy to say "this faction wins in X, this one at Y" etc..., so I will simpy ask one question: Do you want the newest and go shopping every once in a while, lacking models for a lot of things, but wait for them eagerly, or you prefer to make a big buy of 60-90% of the whole range in one go? Also, the oldest units lack the toys and specialization the new ones have, in general.
I started with JSA (okay, vanilla YJ for a slow-grow league and then went full JSA the week after) because I was attracted to the sculpts. The Domaru were beautiful, the Oniwaban in the old starter is honestly still on par with most modern sculpts, and the idea of playing an army of samurai and ninja in a somewhat cyberpunk game appealed to me. Also I'm a fucking weeb, so that helped too. JSA was hard to start with. I don't think I won any games my first few months playing and JSA takes a very different mindset than many other armies do so things other players suggested I try usually didn't work in the way they expected. I kept playing until after Uprising. I put down JSA in maybe Julyish of last year to pick up the now-retired Shock Army of Acontecimento. I picked Acon for some mechanical reasons and some local reasons -- competent line troopers, Aleph specialists, high BS across the faction, lots of access to mines, and a very fieldable TAG were the big reasons, but also because no one local to me was playing it at all. Dart (a new character also available to Aleph and Military Orders) got added to the army about two weeks after my first game with it, so that was cool, and then later that year the army got even better when it got its going-away update. I'm playing Spiral Corps now, which is a bit for mechanical reasons, but also because I was looking into Tohaa before Spiral Corps was released but saw Spiral as a way to get the new hotness. The thing is, having a bunch of both now, I don't really feel like they play very similarly at all. Spiral Corps is much more punchy than Tohaa and has a much stronger reactive turn (and Tohaa already had a pretty strong reactive turn) to which is nice, and Acon and JSA both taught me to rely on a few very good specialists, so playing around Draal and Clipsos feels pretty homey to me. Based on my experience, I would say start with what you think looks cool. Once you've played for long enough to have a good grasp on all of the types of mechanics in the game, then you might want to pick something based on mechanics. But you'll always have fond feelings about your first army, even if you don't get them to the table that often.
I would echo the "rule of cool" approach. The dirty secret is that if you enjoy the game and the hobby, a year or two down the road you will have a good idea what works for you and everything will be painted (haha). Then, you are going to want to branch out into another faction that has features you find exciting once you understand the game and what you like about it a bit better. I would pick something you want to paint up and play today, and know later there are always other options.
@Awash in Blood Do you have 3-5 of your "top" picks, to pare down your list? Maybe cull some factions until you get to that smaller number, as a start. Heck, share that smaller number here later, and we can give pros and cons of one versus another. This is more or less how I picked PanO. They just have so many cool units.