It's hard for me to pick a favorite mini on its own merits, as there's too many good ones to choose from, and that's even with me being as picky as I am.
The ones that stand out in memory are the "landmark" ones in my personal history, rather than necessarily the "best". I got into gaming minis somewhere in my pre-teens, but then fell away from it after just a few years. Then about a decade ago some tangential internet hobby stuff lead to me rediscovering them.
The two the stand out in my memory from when I was a kid (note: pics are not mine):

As a wee trekkie in the late 80s, I loved this particular FASA design because of how perfectly it translated the old Romulan warbird to the movie-era aesthetic. It was IMO the perfect "unseen" counterpart to the refit Enterprise and the K'Tinga. I still feel this way, even though I haven't been a real trekkie anymore for a long time now. I hunted down one of these some years ago, but haven't painted it yet, as I keep redesigning the paint scheme in my head.
A CG version is on my "to do" list as well. I've seen a few CG models of it that other people have done, and the thing that always annoys the piss out of me is how people often omit those tight concentric lines on the primary hull. This is half-understandable, as FASA's own ortho illustrations omit them as well (and CG modelers are likely referencing those without ever even looking at the mini), but IMO that is an essential part of the "look" of the ship, and trying to do a Gallant Wing without that is like trying to do Cajun food without bell pepper.

One of the THE first minis I painted as a kid. Did it it in bright grasshopper-y greens with yellow belly scales and purple eyes (Poly-S paints, remember those?). Sadly long gone (though surprisingly, I still have some of those Poly-S pots, that even more surprisingly are still good). I keep meaning to buy one from Iron Wind (amazing that this is still in production). Still my my favorite "Umber Hulk" sculpt, due to the proportions and shovel-claws. IMO it sells the sense of being an actual big heavy animal (like a rhinoceros or a bear), as a opposed to the more common twiggy sculpts that handwave that stuff instead.

TOTALLY cheesy, but it's the mini that kicked off my rediscovery of the hobby like ten years ago. I had no idea how things had progressed while I was "away". Being randomly linked to the Hasslefree minis site one day and seeing how much better the sculpting was than what I remembered from back in the late 80s and early 90s threw the doors open for me. I was a 1/20-1/6 figure guy at the time, and had long ago dismissed the wee stuff as being too crude and unerdetailed for my taste, Seeing how that had changed didn't just bring me back, it completely flipped my interests. Nowadays I just do gaming minis, no longer big scale figures, because a modern well-sculpted gaming scale mini can have all the visual appeal of a big figure for a small fraction of the price, and a tiny fraction of the display space (1/8-1/6 figures add up FAST, shelf-space wise).