Riv seems to like the Nuk ones the best...or his fist (an ideal scenario for Riv would be if he could fit the dummy AND his fist :P).

With the spitting out the dummy, I find that I sometimes have to hold it in River's mouth til he starts his 'dummy suck' cos he starts with the 'nipple suck' action and then realises 'nope, this isn't working' and changes his type of suck and then the dummy stays in. Good luck finding something that works for you!