Sure there is a simple answer of course but there are several things this slave could do that would be extremely humiliating, exciting and degrading all at the same time. Which would be most humiliating? Also, a disclaimer, by listing these I am in no way suggesting they should be acted upon.
What a long-winded answer to a simple question! In writing this it became obvious that slave was switching these around several times and is probably not entirely happy with the order - but you get the idea. I am sure everyone will have their own order anyway. Strap on - This has got to be one of the most humiliating things a guy could experience. Just the sight of a woman wearing one is a little unsettling and guaranteed to be embarrassing, particularly if the male was on his knees. Everything to do with the use of a strap-on is the reverse of all he has become familiar with and is going against his teaching, genetics, etc.
When it comes to being physically fucked by a woman apart from the obvious but the mental adjustment would be a humiliation and probably irreversible. Not to mention the homophobic connotations it conjures up.
For all the reasons above this has to be No 1. Dildo fucking - similar to the one above something only ever done in private, would be very demeaning to do this in company, because as a solo act there is no hiding from the performance and the pleasure. Similar but not so extreme are butt-plug wearing, including theme plugs like crawling around with a pigtail plug.
Facesitting - On the face of it Ooops! Yes while serving the pussy the slave may also take pleasure in this but it puts him firmly in his place on the food chain sorry.
His humility is absolute and if this includes anal his subjugation is ramped up. A little breathplay here also reaffirms who is in charge.
Chaste Pleasuring - Wearing a Strap on dildo while being locked in chastity. Performing all the usual duties of a man without any of the benefits might be the ultimate head-fuck but is also a selfless act; this one really is all about the pleasure of the woman and the fact; like chastity, he is controlled. Crossdressing - Whether this means wearing panties or stockings or going the whole way with make-up and dressed to the max with killer heels this has to be up there in the humiliation stakes: To be thrown back in your cage is very frustrating and very embarrassing especially after being made to plead to cum and then been denied.
Begging to be allowed to cum takes a man out of his comfort zone and is very degrading. Also being made to masturbate and then clean up the fruits of your labours, from licking it off the floor, drinking it out of a glass or condom or pussy or feet. Nothing is so demeaning as right at that moment the male has been deflated and weakened and the last thing on his mind is to have to consume his own seed like a naughty boy caught masturbating.
At this point post-cumming he is apt to be most rebellious; what better way to reaffirm who is calling the shots by commanding him to lick it all up. Being forced to lie upside down and cum onto your own face or into your mouth and consume or wipe it all over goes to the heights of degradation. Simulated sex - The next step up from the last item is being made to carry out the sexual act with The Domina as observer and not a participant; very demeaning and cringeworthy.
Examples can range from humping the floor, a masturbating device, fucking the legs or feet of the Domme or a blow-up sex doll. This one is only bound by a wicked imagination.
Spanked to Crying - Almost always the things that men find most humiliating they will avoid like the plague so crying has got to be a number one. Be it from spanking, CBT, or any other way most men will go out of their way not to cry. The act of bending over, assuming a certain position or having to take down clothing is full of embarrassment enough to be followed by the blushing and flushing of both sets of cheeks. Then crying from being spanked is a goal for some submissives and though very humiliating afterwards at the time they are past caring, having been cleansed and relieved.
As such this is probably the perfect punishment for inattentiveness or failing to come up to standard as are the use of a dog shock collar around testes and nettles, toothpaste etc. Any of these coupled with an allowed orgasm or oral servitude ramps up the humiliation.
Verbal - The power of speech, especially when in a derogatory or condescending manner it is massive on degradation. Calling into question sexual prowess real or feigned or ridiculing his manhood: Chastity - Almost overlooked in this list of humiliation is potentially one of the most humiliating acts a slave can do, that is to be forced to wear a chastity cage.
This is a constant humiliation for him everywhere he goes;the realization that he is no longer in control of his own orgasms; also knowing that His Goddess is also aware of his predicament. Yet he loves to be kept in chastity; this perhaps confirmation that any humiliation is all in the mind.