I have a couple of favorites which really stand out to me.
First, I am a submissive bottom and while on my life's journey into the realm of BDSM Fetishes I first found a tremendous addiction to spanking as an erotic punishment. But it wasn't until I was put into bondage to receive the punishment that I found a truly more erotic sensation of being unable to control the scene. This greatly enhanced all the normal feelings of being spanked and punished, the humiliation, the vulnerability, and now coupled with the feeling of fear or the unknown! Knowing the inevitable that I WILL be punished and there was no way to stop it and now adding the fear of not knowing what spanking implement would be used, or how many swats I would receive, or how long the punishment would last, or how intense was the punishment going to be! All of these thoughts would be combined to create such a heightened euphoria of emotions and the outcome was that I was able to endure even the harsh and severe punishments of multiple spanking implements and even the cane!
Another fetish combination I have come to enjoy is again after being placed in bondage, usually for this one, a kneeling hog-tie. I am on my knees, then my face and chest are forced flat on the bed or floor, my knees are spread wide, my arms and stretched out under me between my knees and my wrist cuffs are secured to my ankle cuffs which are also sometimes attached to a spreader bar although in this position I have very little ability to move even an inch. A spanking punishment opportunity is never wasted when I am in this particular position with my ass raised up so obscenely but oh so inviting to be spanked, paddled, whipped, cropped, and caned. The perfect Fetish combination to this now well marked butt is a nice ass fucking. I have had the pleasure of serving a Sadistic Dominant which had incorporated both spanking and anal torments as my Obedience Training. So for me, when I was to be punished for anything, it usually consisted of some both fetishes combined as one punishment session, or a couple of small errors or poor performances on my part were added up and dealt with using each of the fetish punishments while I was restrained in bondage and unable to escape either or any of them!