Boy, there are so many good choices and possibilities...
Splash Mt. is a definite "must-ride", but it may be "dangerous" to use it for setting expectations.
The things I'd offer up are:
Peter Pan/Winnie the Pooh - well done, easy rides. If you are familiar with the movies/characters, the rides will be very "familiar" and will definitely tell you that you are at DIS.
Jungle Cruise - nobody has offered this, and I can see reasons why (needs updating, lines can be long, experience depends on your 'skipper', etc.). However, the queue is cute, with the different elements about missing expeditions, and the "expectation" that it might set won't be difficult to exceed with other rides. Still, despite cheesy jokes and some of the oldest "animatronics" known to man, it is very "Disney".
The potential problem with all three is the possibilty of long lines. I think Peter Pan is awesome. Unless you're a big Pooh fan, Pooh is not worth a LONG wait, but it SHOULD be enjoyed with a fastpass or 15-20 minute line at some point. Of course, that depends on your personality and what you enjoy...
Philharmagic is great, too. Good thing about it is that you will probably not have a long wait outside. In less busy times, I'd guess that the theater accomodates all the people who are in line (though some people here surely know better than I). If you're going to WDW, I'm guessing that you (or significant other) have SOME exposure to Disney and probably have favorite characters, etc.. Philharmagic seems to run through most of the popular pre-Pixar characters, and you're bound to see your favorite at some point.
I'm being long-winded, but I'd say that the first ride depends on how you want to approach it. If you want one of the best (and longest) rides Disney has to offer, Splash Mt. would be my choice. If you want to start at a very-Disney-but-a-little-dated ride, Jungle Cruise fits the bill. If you want somewhere in the middle, Philharmagic, Peter Pan, or Winnie the Pooh would be good. I guess Small World is right there with Peter and Winnie. It's VERY Disney (what Disney reference can you see outside of Disney itself that doesn't say something about it?), and probably sets a sort of "medium" expectation for future rides.