I'm not the biggest one for reacting in the theatre. I think it's the years of "sit still in chair, remain quiet, and everyone can hear the show" that have been pummelled into me. Genuinely hilarious stuff doesn't raise much more than a titter. When tears start streaming down my face, I keep my mouth FIRMLY shut so no embarrassing "SOB!"s emerge. I'm very sparing with my standing Os.
That being said...
Laughed 'The 25th Annual Putnam County Spelling Bee'
'The Drowsy Chaperone' ('Bride's Lament'; oh god, I hadn't even heard the cast recording when I went to see the show, so this part just killed me XD)
Cried 'Bent'. Yeah, who'd've thought that a play about homosexuality and the Holocaust would've had such a downer of an ending? :P
That... might be it. I know I've laughed and cried during plenty of shows, but these are the only ones with such a large reaction as to be notable. :3