I don't have any big models that stands out as a center piece but I have a few models that I personally like as my favorite of each army.
Orks: Its a toss up between my Mega Armored Warboss (Vankraken) which is a beast of a tank and dead killy (love the Kromlech model). My Flash Git Kaptain with his cybork snazzgun arm is probably my favorite kitbash/paintjob followed shortly behind my Tankbusta Nob with his Rokkit Launcha "Fishbonez". My Dakkajet "Da Danga Zone" piloted by the legendary "Kenny Waaaghins" might qualify as its the center piece of my Ork flyer wing.
Tau: My Iridium Armor Commander would probably qualify but he isn't all that special of a model and not a very particularly well done paintjob. My Ethereal and Darkstrider is probably the most creative of my Tau models but one is a simple conversion of a Fantasy Battle Mage while the other is a pathfinder conversion. Maybe when my one and only Riptide is complete then it will become the center piece.
Grey Knights/
Inq: Probably my Librarian as its usually the
HQ and its a decently well done model. Soon it will be my Malefic Inquisitor Mannis (conversion from the chaplain character from the Calth Box) which in my own head canon is the leader of the "Fiery Heart of the Emperor" army.
Space Wolves: My Wolf Lord (Bard) on Thunderwolf is probably the most impactful and stand out model of my army but I have a soft spot for my Wolf Banner Grey Hunter. Probably my best paint job on a fairly rank and file model.