I love Bruko ukes (I have a number 5 that was my first 'proper' uke, and old maple 'sunburst one' and a concert I recently bought on the Marketplace here). They're ridiculously good value, great workmanship, sound great. They can have a slightly higher action (and you can't easily adjust it due to the one piece bridge/ saddle arrangement on their standard model sopranos), but it's certainly well within my personal limits of what I consider too high. It's probably why it's one of my louder ukes, too.
No experience of the Martin S1.
The Pono sopranos (despite the fact that I love their concerts, tenors and baritones) just never did it for me, though they're certainly well made, nice playing ukes.
I'm not really contrary but, although I think the Brukos are the best built of the three, I would prefer the Pono and the Martin. If you like the sound of the Bruko, as some do, it's the best choice. They are consistently far too bright sounding for my taste. I hope you will play all three before deciding.
I've added Worth Browns to my Bruko #5 to mellow it out. It was a little bright. Spectacular ukes. Great sound, very playable.
I also have a Martin SO, which is different than the S1 due to the extra frets (maybe more differences I'm unaware of) and it sounds and plays great. The action on that one was a little high at the saddle and because it's glued in and hardwood, I was able to take off the strings and lower it a bit. Made a lot of difference to me. I now pick it up much more than I did. It's very sturdy. I have no worries about breaking it anytime soon. But, it still sounds very nice at the same time.
I would try to play the one I bought, but if I was ordering them without playing, I think I'd have more confidence in Bruko. Just because of how meticulous you always hear they are.
Save up, get one of each!
I have no experience with Ponos.