Hi! Here is my two cents to help with your decision...
Malachite is the most gorgeous of all your choices. Visually, it is most stunning, the one that pops the most. I love it! The only thing about malachite is the stones that touch your skin may turn color. This is especially true if you put lotion on your body, including your neck. If you wear a scoop neck (no collar) top, you may have the back two motif lay against your nice silky skin and pick up oils. If it doesn't bother you, I would get this combo. Malachite in YG (not WG) is beyond gorgeous to me.
If you feel you have to baby your malachite or will rarely wear it, skip. No matter how gorgeous a piece of jewelry is, it's not doing any good sitting in a jewelry box.
Lapis is my next choice. Lapis is stunning and looks so rich with YG. VCA now has amazing lapis; it is top quality and so pure blue. Lapis is also super versatile, casual, dressy, all seasons. Blue is a universal color, for all outfits and seasons.
To me, malachite is the most stunning, but lapis it the most practical. Both are beautiful in its own way. Stunning or practical? As they say, pick your poison!
Oynx is lovely, but it can be harsh. I also think black is not year round, at least not for me, where summers on the US East Coast can be 80+ degrees and humid. For summer, I wear shorts and light colored clothing, hence I think black oynx is a bit harsh/seroius looking. Think about if this would fit with your outfits.
Guilloche is too much metal for me , so I would not consider it. I see it as a long metal necklace with a few diamonds sprinkled. For me, I am not into the bold, shiny metal, chain necklace look. The diamonds in the motifs are too small to call it a "diamond necklace."
Last is severe porcelain, and I also would not consider it. I am not a fan of porcelain. Jewelry is 100% preference, and for me, I prefer motifs to be made from natural stones, each unique, so I can admire the beauty of the natural stone. I do not see porcelain as jewelry for me; dinnerware, tea sets, limoges, all yes... but not for jewelry. That's just me.
Malachite is just stunning! I love green stones. I'm a big emerald fan, and I will say, I get the most compliments when wearing bright happy gemstones.
Since you listed lapis first, I think you will like lapis more than malachite, so I vote for lapis for you!
Again, this is just me, and everyone has different tastes. I just wanted to share for another point of view.
Good luck in your decision! It's so exciting to have so many choices!