I'd say the oral ROA would be the easiest to get away with/least bother, but I guess anything is possible with zip pockets
I'd say snorting is dodgy because of the risk of a polo and the general runny nose/sniffing.
Smoking for the smell/risk of setting off smoke alarms in the toilet etc, id say carrying a glass pipe on you isn't exactly easy, maybe if you are a woman and can get away with carrying your handbag to the toilet every time you want to go.
Shooting, you'd have to carry quite alot of stuff, but could be pretty stealth, especially with pre loaded rigs, can be completely silent. Apart from the sound of lightning shooting up your spine and coming out of your ears of course
just be careful of blood spillage and cleaning up paraphernalia. I do this in public toilet cubicles with others in and out all the time to no ones knowledge, even in hospital on the ward only behind the curtains with nurses working on the other side, jus give a sly but loud throat clearing noise or cough as you spark that lighter, and always place your spoon down onto something softer than a floor. Maybe cough when you flick the bubbles to the top too, or learn how to shake them out.
Plugging, like
@Perforated mentioned, id say is pretty stealth too, could be near enough silent and very stealthy in a toilet cubicle, i wouldn't risk doing it anywhere else though, particularly hard one to explain
carrying a little vial of say 20ml of clean water(as with shooting) would be very helpful and save any risk of being seen filling up from a tap or something.
So yeah oral or sublingual gets my vote for ease, id personally choose shooting or plugging over snorting or smoking too.
Oh and get some magnesium(is it?) for the whole jaw clenching thing