The OP makes the quite false assumption that I have only one conlang, but nevertheless I have an answer, provided we construe the question as simultaneously being "what's your favorite among your conlangs and what about it makes it so?".
. While not strictly adhering to features attested on Earth, I feel the more interesting bits are still all reasonably plausible, and I feel that they represent some of my best conlanging ever. Wrapped in a phonology I have yet to tire of. (The syntax of Jamna Kopiai is also delicious, but has the serious obstruction of requiring book-length exposition to get one's head around it. Whereas reading the Tlaliolz grammar is a breeze so long as you've got a good grip on the terminology and grammatical concepts.)
What's particularly good:
1. A robust active-stative fluid-S morphosyntactic alignment in which I've merged the notions of morphosyntactic alignment and grammatical voice into one unified mechanism, such that our terminology for either becomes somewhat interchangeable with our terminology for the other. Note that this is also true of e.g. direct-inverse languages like many of the Algonquian languages, but I've done it in a different way that I feel is still reasonably realistic.
2. "Compound verbs". Read the description in the link. Essentially, it works exactly like serial verbs do, except the involved verb roots 1. are morphologically bound to each other and 2. share the same clausal arguments between them.Aqq’omēq’uni lia.
go-stab-kill-PFCT.1.SG.NOM 3.SG"I went over and stabbed it to death."
3. "Serial clauses". They're not coordination, they're not clause-chaining, they're somewhere in between. And they work hand-in-glove with compound verbs: whenever one element of a compound verb must be independently modified, it is a simple transformation to break it out into a serial clause so that it can be. This matter of one piece of creative grammar shoehorning beautifully into another is a rare gem among my conlangs, which tend to be many-headed syntactic hydras where all the bits don't entirely fit together right.
4. A strong tendency towards employing morphological binding of multiple roots for grammatical purposes, displayed prominently in both a) the compound verbs and b) the fact that attributive modifiers are generally incorporated into their heads. Despite Tlaliolz not being an especially polysynthetic language in other ways. This may be the least realistic thing about the language, but I don't care, I'm keeping it. It gives Tlaliolz a lot of its character.
It's been three years since I last did much with Tlaliolz, mind you. And at the time I thought I was just making a weird little toy language as a diversion from bigger projects... but as time has passed, I have more and more come to appreciate that it's just possibly my best single creative work. So I'm proud of it.