Mmm, picking just one is difficult. I’d say if I had to sell off my collection I know the last thing that I’d part with, so I guess it would be my favorite.
The eFx PCR Darth Vader helmet. There are higher quality (and much higher priced) Vader helmets out there, but for the money this thing is spectacular. It’s a replica of the original 1977 helmet and it has a well documented lineage that traces directly back to that original screen-used helmet. Basically, some time after filming Rick Baker cast a mold of the actual screen used helmet (probably in 1978) and that mold was digitally scanned and used as the basis for this prop replica.
Other runners up would be my Hot Toys ESB Darth Vader 1/6 figure, my Mafex Boba Fett figures (both ESB and ROTJ versions), and, of all things, my micro machine collection. I always loved Star Wars micro machines and when they started producing them again when TFA came out, I dove in hard. I was quite disappointed when they canned the line again after R1 but I’m glad we got what we did.