Well, you've started with some big names. First off though, as someone who has been at this frag game for many many years, I urge you to get rid of the idea of "better." There is no objective better in the fragrance world; what really matters is how well something resonates with you, and how well it projects the image/story/impression that *you* want to project to the world. Those are the only things that matter, so don't get caught up in price, prestige, or the opinions of "better or best" that you'll undoubtedly get galore from the fragrance community. For a random example, I enjoy my super cheap SoTD Ralph Lauren Romance Silver far more than say, Opus V or Creed Aventus or any other number of scents that cost ten times as much as it did, and no arguments of objective quality (as if the average fragrance consumer can really judge that?) will make me enjoy Aventus more than Romance Silver. It just is what it is.
I say that because I've honestly seen people get swayed away from scents they love because the scent didn't get the "fragrance community seal of approval" - and that makes me sad. You shouldn't have that issue with Opus VI; it's been very well received, but you will surely come face to face with the issue at some point and I hope you stand your ground and just love what you love.
That all being said, I've not tried the Mona, so I don't know how it compares. I will say that you might want to give Bond's New York Amber a sniff, as it shares a number of qualities with Opus VI (a dry and yet fairly lightweight amber), but has a number of differences that set it apart. It's more floral in the dry down, and more dry and astringent in the opening, and has a bit more evolution. I personally prefer Opus VI, but I do think that the Bond is worth a sniff as it is honestly the closest scent I've smelled in comparison to VI. Most ambers are thicker, heavier, more dense or opaque, sweeter, and less dry than Opus VI.
If you want to try the absolute 'reference amber' - imo, as it really just captures the core labdanum/benzoin/vanilla accord that IS amber - do sample L'Artisan's Ambre Extreme. If you compare Opus VI to it, I think it should be pretty clear why I chose the adjectives I did for Opus VI above, and using those two points of reference (and the Mona), people can help you determine just what qualities you are really loving, because "amber" as a category is pretty wide and encompasses many different scents with drastically different qualities.
Edit: Should also mention that I'm not a huge fan of amber oriented scents in general, but I do love Opus VI. As mentioned, Opus VI and New York Amber are pretty unique ambers - liking them does not necessarily equate to liking amber scents as a category. That's why I do suggest trying out a more traditional amber such as the L'Artisan mentioned above, or maybe Montale Blue Amber - those are much more representative of the typical amber accord.