This is like asking a parent who their favourite child is. Ya know, out of 2023 kids.
There's a link to photos of my collection in my signature panel (albeit old and out-of-date photos now )

But okay, lemme try favourites by year...

+1984: Soundwave
+1985: Beachcomber
+1986: Ultra Magnus
+1987: Apeface
+1988: Quake
+1989: Countdown
+1990: Ground Hog
+1991: Gripper
+1992: Hurricane
+1993: Deftwing
+1994: Megatron
+1995: Laser Optimus Prime
+1996: Optimus Primal (Ultra)
+1997: Airazor
+1998: Megatron (Transmetal)
+1999: Archadis
+2000: Super Mach Alert
+2001: Blast Punch Optimus Primal
+2002: Runway
+2003: Unicron
+2004: MP1 Convoy
+2005: BT Overdrive
+2006: Classics Bumblebee
+2007: Leader Class Optimus Prime
+2008: Henkei! Sunstreaker
+2009: Leader Class Optimus Prime
+2010: Bludgeon
+2011: United Frenzy
+2012: MP Alert
+2013: MP Soundwave
+2014: MP Ultra Magnus
+2015: Combiner Wars Megatron
+2016: MP Ironhide