All infant seats tend to get good crash test results probably because the design of them is so simple and they are all rear facing.
If you want to absolutely maximise safety then I'd go for a highly safety conscious market leader, probably Britax or Besafe. But arguably, you don't need this - as long as you don't get one where the side impact protection is nonexistant (which are very unusual these days anyway) a cheap/basic seat is just as safe as the most ££££ one - the ones which recline tend to get a very slightly worse score for crash safety, compared with the ones which don't.
Remember that a car seat is only as safe as promised if it is used 100% correctly. So ease of use is very important if you're trying to optimise for safety - another reason to prefer simpler models over complicated ones. IMO, Britax and Maxi Cosi tend to be designed the best for user-friendliness (though I prefer Maxi Cosi's simpler seats e.g. the Pebble Pro is a near-perfect seat).
In terms of raw numbers:
The best ever scoring, currently available one on ADAC is Cybex Cloud T, fitted with seatbelt (no base). This scored 1.0 (lower numbers are better). I would highly suspect that the difference in score on vs off base is related to the fact that the seat is only allowed to be used in the recline position when on the base - it can be fitted in the car with seatbelt only in the upright position.
Scoring 1.2 you have Nuna Pipa Next / Nuna Pipa Air, each when used with the "wing" base. However some people find the positioning in the Nuna seats to be quite scrunched up for baby and aren't generally happy with it.
Then a whole slew of seats scoring 1.3 for safety. I could do with updating my list so I'll collate them in a minute.
It is worth noting that Cybex are fairly known in the industry for optimising specifically for that crash test. It doesn't mean their seats aren't good, they are good, high quality seats. I had the Aton 5 (second best ever scoring seat, but no longer sold) for my younger two and I was happy with it. But I think you should always take exact numbers with a pinch of salt. Realistically, anything with a safety score of 1.5 or better is an excellent seat. Also depending on when your baby is due you might want to wait for the October ADAC test results because I expect they will test the Besafe Go Beyond which is a new seat which has some new features not seen before.
I also didn't include seats which cover several age stages because it is hard to break down what the result for a newborn is vs the result for an older child. It might be that the Which? results do separate these out - I don't have a subscription currently. Another general rule aside from keeping it simple and easy to use, is to stick to seats which do one job, and don't try to combine two roles (e.g. ERF seat and newborn seat) into one unless your use of the one "mode" will be very brief or minor, so the convenience of the two modes outweighs any downsides of it.
There was a test run a few years ago by a safety organisation in Norway, I think they were trying to establish some kind of version of the Plus Test but for infant carriers. Besafe izi Modular X1 did best in that test. However they were criticised by some other safety experts for the test being OTT and irrelevant, and it's never been repeated, so I am not sure if it's useful information or not.
Are you wanting safest in terms of crash safety, or best breathing position etc for the baby? To an extent you are splitting hairs between different models of infant seat particularly for the crash safety angle. Breathing position there can be some differences. However this is unlikely to matter if you stick to the standard guidance on not keeping baby in the seat for more than 2 hours at a time (30 minutes in the first 4 weeks).
You will also want to get the seat assessed in terms of how well it fits your specific car. The easiest way to do this is find a local car seat shop (preferably an independent) who will try out different models in your car for you. Again it's no use buying the safest seat in the world if it's incompatible with your car.
As a PP said, the best thing to do in terms of safety is, for the next seat, ensure you're likely to rear face for as long as possible. For the infant seat it matters less because they are all rear facing anyway. You might actually want to look ahead to see what seat you want for the second stage and then kind of work backwards from there, ie see if the seat you want for the next stage can work from newborn, if it needs a base and if so if there's a compatible infant seat for that base, what the minimum age for the next stage seat is and therefore how long you need an infant seat to last for, or how much your favourite next-stage seat costs and therefore how much you want to spend on the infant seat.