My favorite November monthly collectible would have to be the Ice Gauntlet.
That's... a little unusual, I think. Even though it's one of the older collectibles out there (and, by association, one of the rarer ones), it doesn't fetch a terribly high price on the marketplace. And, honestly, I can see why: the Ice Gauntlet, like a lot of other older collectibles, isn't as flashy. It has fewer options, and most of the options it does have lack a certain polish that more recent collectibles all seem to possess.
But it's still my favorite November monthly collectible. I might be less likely to actually wear one than some newer things, but the Ice Gauntlet holds value for me out of a kind of nostalgic sentimentality.
I first joined Gaia in 2004, and while monthly collectibles -- donation items -- weren't quite new at that point, they were definitely a great deal more... revered, I suppose, than they are now. They were the biggest items on the market, and you'd almost never catch one selling for less than something that wasn't a collectible. That's... less true, these days. Largely untrue, I suppose. They still go for more than most gold-purchased items, usually, but between cash shop items and game prizes, the competition is pretty stiff, and the monthly collectible isn't quite the undisputed lord of the gear that it used to be.
Only the oldest items are uncontested kings of the market. I feel pretty stupid for having sold so many of mine, back in the day. Huge mistake. But, then, the 40,000 gold or so that they tended to sell for back then seemed like a lot at the time. Even the Halo only went for a single million or so, in those days.
It's not just that it was 2004, though. The Ice Gauntlet came out around the time that monthly collectibles were first starting to be wearable in different ways. It wasn't the first, of course, but it was one of the first, and I vividly remember playing around with it until I settled on a favorite appearance: one of the partially-extended-arm settings where the ice formed claws. It was new, back then, and really special. Unless I'm remembering wrong, nothing except for collectibles had that kind of freedom back then. The Ice Gauntlet reminds me of a time when those options were rare and precious, and while it's probably good that it's become more common since those days -- it allows for a great deal more freedom in setting up one's avatar, after all -- that time was special to me.
So... Maybe it is just that it was 2004, but if so, it's still not just that it happened to be released at that time. It's that it represents something about that time, to me, and it's a something that means a great deal to me, personally.