Perhaps it's because they were on the cover of the very first hardback D&D book I ever purchased or maybe it's because they are such awesomely complex monsters with so much going on. Whatever the reason, I have always loved the githyanki and can count them among, if not at the top of my list of AD&D monsters.
In my younger days, I lost my favorite barbarians character, Balor, defending a thieve's guild hall against a githyanki onslaught. He fought valiantly for nothing more than his honor and his word, but could not stand up to the combined might of githyanki warriors, their magic, and the red dragons they rode in on. When the dust cleared, Balor was dead, but I had an epic battle to sing about in that braggadocios way that only a young, elementary school aged gamer can. The thieve's guild fell but the thieves, my friends' characters, were able to escape with the silver sword that the githyanki came for. Later, Balor's son would wander into the world of the civilized and take up a quest for vengeance.
In my later gaming career, I toyed with the githyanki a lot. They were secret agents, dark assassins, minions and at one point the vanguard for the coming apocalypse of my gaming world. In this form, they were not the githyanki, but simply gith, still in the thrall of the ilithids. They were stormtroopers and hunters and I even came up with some strange, twisted ilithid technology for them to use, including a subtle nod to my love of mecha in the form of magical, symbiotic, insectoid armor (idea borrowed from Aura Battler Dunbine and Spelljammer). The gith minions were not as individually potent as githyanki, but then again they were still minions. However, with the tools provided by their masters they were dangerous in new and different ways.