I have now owned the SLC, the Conquest HD and the Leica Trinovid HD. I have not yet had the pleasure of using the Monarch HG's, although if/when I do, I expect to like them very much (I've always been impressed by Nikon optics).
Of the four, I chose the Conquest HD's. I had the SLC and Conquest at the same time, and I could not see any difference in image quality over multiple tests in many lighting conditions. They are both built very well. I think what you're getting for the extra price of the SLC's is the outstanding Swarovski warranty. They will take care of you no matter what happens to your binoculars. So that is worth quite a bit to many people. For what it's worth, both pairs of SLC's I owned had pretty annoying focusers. Dry and sticky. Shockingly bad for the price.
I have used the Zeiss warranty once, years ago and it was an "okay" experience. They took care of me, but it wasn't particularly fast or convenient. But I was satisfied in the end. I have used the Nikon warranty twice in the past 10 years, and they also took care of me eventually, but it was really slow and I had to send one pair back twice before it was actually fixed.
I can't comment on the Leica warranty other than I've not heard great things about it unfortunately. But they are lovely optics, especially if you wear glasses. Field of view on the Leicas is a bit disappointing though.
I would say you just need to prioritize what's most important to you. If it's size and weight, the Leicas and Monarch HG's will win there. If it's field of view, that goes to the Monarchs. If it's a bulletproof lifetime warranty, that goes to the SLC's. If it's a great focus wheel, then the HG's will probably be best.
They are all great, but all slightly different. If you can figure out what specifically you want most out of a binocular, that may help you decide.
One last thought - of the four, the SLC's will hold their value the best, followed by the Conquests, then the HG's, then the Trinovids. I think there's a clue in there somewhere about what the consumers think of those four.