This was originally a 3 sentence response, but as I thought about it more to organize my thoughts, the answer got way too long. But again, would rather think this out now than try 20 separate actions. Small text where the answer is too long.
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For custom actions - I almost don't want to say the ones I didn't like. This is because I only had a sample size 1 or 2 and can't rule out bad units, the smith/assembly (and not the action mfg) goofed, bad rounds/loads, I goofed, etc.
Anyways - I'll have to dig up my notes to be sure but IIRC of the ones I didn't like one was a curtis as it felt like the bolt was always just about to bind while I cycled it. I think a bighorn I fired 2 or 3 rounds from had bolt lift/close issues (no flat primers, cratered primers, etc.). Another shooter who was there complained about how his action wouldn't cycle reliably if it got even a little bit dirty / dusty but I don't remember what action it was and I didn't try it myself. I also remember trying one ARC (think it was an early mausingfield per the owner, don't remember off the top of my head right now) that just didn't feel well put together in the way something feels when all the tolerances are just a little bit off but I tried another mausingfield once and it was WONDERFUL, not sarcastic. I think I also tried an impact and lone peak fuzion - the impact "just felt good" all around even though I wasn't used to the cocking being split between bolt open / close. The fuzion I guess was analogous of a Tikka for factory rifles - IMO nothing in particular wowed me, but the thing just felt like it worked in every single way, a little more so than I felt with the fuzion, which I guess should wow me on it's own. There were a few others I don't recall right now. I haven't tried Kelby, BAT, or Terminus as far as I can remember and I know there are many more that I haven't listed here.
Can list some likes / dislikes about most modern factory actions too but excluding as post is already too long.
Regarding some individual prefernce stuff - there are a few things I very mildly prefer (i.e. screw on rail and certain finishes), some things I don't care about (integral vs pinned recoil lug), some things I just won't know 100% until I get the individual rifle built (magazine configuration), but generally speaking, for most features I can't say I like one thing over another as a "checkbox item". Rather, it comes down to if something just "subjectively feels right or better". I guess this is why I'm looking for a place to try the actions. Otherwise I guess it would be really easy to just make a spreadsheet of features. Customer service is important too but I'm still going to pick a product I like over CS unless it's well below par. And hopefully the company is at least somewhat financially healthy but I doubt anyone really knows the answer to this. Here are just a few examples, not exhaustive:
60 vs 70 vs 90 degree bolt throw - hard to say, I have or have shot other factory and custom rifles with varying bolt throws - for me it wasn't so much a preferred angle, but some bolt throws just "felt better" than others when lifting / closing regardless of degrees of bolt throw. For example, I don't particularly like how the Tikka T3/x bolt feels when cocked where it starts easy, gets harder, and then suddenly cocks - I find that sometimes I goof, my fingers slip, and I have to do it again. Also it throws the rifle off target a little more. However, a Weatherby mk V with a shorter bolt throw actually feels pretty good to me and I don't get thrown off target much either.
Cock on close vs open - Everything modern I've tried is cock on open, but if I think of an Enfield vs a Mauser or clone, it really came down to the individual rifle.
Smoothness of bolt rails and wiggle in bolt - For example, Tikkas have some wiggle in the bolt due to generous clearances and the bolts / rails aren't super polished by any means - you actually hear and feel the bolt dragging against the rail the whole time, but somehow, when all is said and done, it seems like most people feel that they're still pretty smooth and easy to cycle for a factory action and almost impossible to bind. On the other hand, I have tried a lot of other factory rifles with tighter clearances that were more polished and better finished, but when I actually cycled them, they didn't cycle as easily (and they weren't dirty).
Extractor / Ejector - Mildly prefer the pseudo CRF of the Sako 85 but for the 85, I think the ejector placement at the bottom of the bolt face is also really dumb as once extractor spring loses tension, the casing tends to eject up into the scope / scope rail (i.e. jam the gun), especially on longer / heavier cartridges unless there is one round left in the magazine to help push the empty casing out properly. Like it mildly more than other actions as a CRF style ejector gives some control over how the casing is ejected which is convenient for collecting brass, mostly controlled feed b/c controlled feed benefits, but the extractor design still allows me to throw a round in and close the bolt as needed unlike a Mauser or other CRF actions I can think of.
Prefits - yeah, this would be nice, but it seems like everyone is trying to offer this on paper now. I watched one rifle get a barrel swap - this was not with a pre-fit barrel and the smith performed some minor fitting on par with what I expected. The other was with a different mfg rifle with a pre-fit barrel and based on what the smith told me, he performed about the same amount of minor fitting as the first rifle. So I don't know if, practically speaking, I'm going to need to see a smith every time anyways to "really do it right" or if I just got lucky / unlucky with those 2 rifles, my smith was just being really picky, etc.
Interchangeable bolt faces - I don't know there will be some practical limits just due to differences in centerfire cartridges and bolt face sizes, but I don't knw enough about the minutia of rifle action production, clearances, and tolerances to know if this is a good idea or not in terms of reliability, cost to properly implement, user safety, etc. vs whatever other solution someone may come up with.