Hi Phil,
Thank you for asking us a question, we all love helping everyone plan their trips :-)
I've stayed at Port Orleans Riverside several times and my family loves it there, it's a very pretty resort with a fantastic food court and the water taxi to Downtown Disney is a great perk. Which building you request really depends on what is important to you and which style of room you like. You can view lots of photos and information, including a map, at the fantastic website
www.portorleans.org
You'll see that the resort is split into 2 distinct sections; Magnolia Bend which resembles big Mansion Houses and Alligator Bayou which have a very rustic feel to them. Once you decide which you prefer, I personally like the Mansion houses better, you can then choose your building. POR is a rather large resort so if you'd prefer to be close to the main building where the restaurants are then I would recommend you request buildings 14, 15 or 18 in the Alligator Bayou or 80 or 85 in Magnolia Bend. If you'd like a quieter room and you don't mind a walk, and to be honest the furthest building is only about a 10 minute walk away, or you have a car then any of the outer buildings would be ideal.
You can make your requests about 3 days prior to your arrival either by faxing 407-934-5777 or calling 407-934-6000.
Have a great trip,
Jo