Some people think Ranma does not have a very good memory. I disagree; Ranma has a very good memory. Ranma has demonstrated the ability to recall things that happened many years earlier such as remembering losing the Tendo Dojo to the Gambling King over 10 years earlier (which even though I dislike that story arc; I find to be pretty impressive after all how many specific events can you remember that happened 10 years ago not counting major life changing things like a death or wedding). Ranma remembered other things that have happened 10 years ago as well such things as messing with Ukyo's sauce. He was also able to effortlessly recall many instances of Genma stealing food from him, events that happened years apart. He also remembered where Ryoga's house was.
How can I say Ranma has a good memory when there are two instances in which he did not remember things? The instances of him not remembering are; not remembering his mother and who Ryoga was. Not remembering his mother is very easy to explain. In the manga; Genma took Ranma away when Ranma was a baby not even able to walk yet. In the manga; Ranma's earliest memories are training in a foreign country.
Not remembering Ryoga is a bit harder to explain but not that hard. Ranma was able to remember Ryoga after thinking about it for a few moments so while he did not remember him instantaneously he did remember Ryoga. There are many possible reasons as to why he did not remember him instantly. Things like Ranma may not be good at remembering people's faces. Ranma had absolutely no reason to remember who Ryoga was nor did he expect to see Ryoga again. Ranma and Ryoga were not friends, at least not close friends, in middle school; Ranma bounced off of Ryoga's head grabbing the last piece of bread that the lunch lady would throw into the crowd. It is likely that Ranma was not in the same class as Ryoga. Ranma probably was only at that school for a few months and Ryoga, almost positively, missed several days by getting lost. My question to you, dear reader, is: How well would you remember someone you interacted with for a few times a day for a few months over a year ago after many stressful and important things have happened (in Ranma's case trip to China, hunted by Shampoo, started going to a new school, obtained a curse)? Lets say you bumped into a teacher who subbed for a few months in one of your classes, over a year ago, how quickly would you remember them? Before someone say's but Ranma acted like Ryoga was his friend. Remember this is Ranma a person who considers just about everyone to be his friend and in my opinion, and the Memorial book's chart, even sees Kuno to be a friend (or at least a sparring partner).