Which would you pick and why?

I really want to like that hydroconquest cause I’m a big fan of longines. Just something about the missing 3/date window/dial balance that I can’t get past. That Oris looks perfect so gets my vote.

I own a smaller Oris Aquis, the 39.5 mm and i have to tell you that it wears brilliantly. That bracelet is truly a work of art. My vote goes to the oris, although i do really like the hydroconquest on the rubber strap as well. 

I have a hydroconquest grey dial and an Aquis green dial.  Of the two I prefer the HC. The HC has a much better power reserve, keeps much better time and wears thinner on the wrist. I also prefer the dial. 
I don’t know about the Certina so difficult to comment on that one.  

In terms of design, tech and diver bonafides I think the Oris wins this comfortably.

I'm not wild about any of these three brands personally, but based on my gut, and the feelings of YouTubers I respect, I think you'll do best with the Oris in this example.

i dont know why but i prefer oris

I have been blown away by the hydroconquest since purchasing on a whim, as I picked it up for a £1000 👌 Build quality, accuracy, feel on the wrist ✅✅✅this is the 43mm

I have an aquis 43 blue dial. It wears great even on a small wrist like mine. The looks on it are perfect and the only negative thing I see about the watch is it is a smudge magnet with all the polished surfaces and the ceramic bezel insert. I doubt the relief insert will have that issue. I have no regrets buying it and I intend for it to be my last diver

I have an Oris 43, and I think it will wear larger than the HC. I like mine but the best I can do is narrow it to those two and suggest you try them on your wrist before you buy

I have the Hydroconquest 41 in blue, and the Longines is definitely my favorite of the three. I expect the Oris beats it (and the Certina trails it) in terms of overall quality. But, almost nothing about watch buying is rational. Go with the one you think you will enjoy wearing and looking at on your wrist.

I own the Certina and as good a watch as it is (and certainly far cheaper than the others) I don’t think it’s the same standard as the Oris or Longines.

I’ve got an Oris Propilot and the build on that is first class. Of these 3 in the quality stakes I would choose:

1. Oris
2. Longines
3. Certina

Oris cuz i ❤️ Oris 😂

I’d say Oris first and coming in at a close second is the Longines.

Of the 3, my personal choice would be the HC.  In 41mm. I have owned a few Longines, its hard to argue the quality of these watches.  However all 3 are solid watches. Buy what speaks to you. Look forward to the pics of the new purchase.

Oris, with date window at six it looks much more balanced and other design choices they made speaks to me more.