Characters are usually memorable due to personality traits. If we think back to all of our favorite characters as we grew up, you may even notice a pattern of characters you liked in comparison with their similar traits. For me, I always liked leader characters. Tommy from Rugrats, Leonardo from Ninja Turtles, Aragorn from Lord of the Rings and so on. You notice that all of MY personal favorite characters that are memorable have similar personality traits.
What makes a character memorable is how easily someone can relate to this character, whether by similar life stories or by personality traits (whether they are common or traits wished to have). The hard part about memorable characters is that, not everyone is going to remember a character the way you want. You might remember a character for their heroic deeds or the way their family interacted reminded you of home, where as I might remember a character because they shared the same life story and struggles.
If you are looking to make them stand out, give each one of them a quirk. Maybe 1 of your characters doesn't take the adventure as seriously and is the "comedy relief". He would be memorable for his quick 1 liners at the right time. Maybe the other guy does a feat during the peak of the story which pulls on the emotional strings.
Ultimately, you want to make each character well rounded and have a good deep development. If all we knew of Aragorn is that he was some guy who lived in the woods, fought, and then became king, his character would be fairly dull. By adding in background information, mystery, and character development even within the story, his character then becomes more in depth, more relatable, and in turn more memorable.
Think about some of the adventure stories, The Hobit, Lord of the Rings, Redwall, Game of Thrones. What makes characters stand out to you? Especially in a story like GoT where there are so many characters, what makes one stand out over another? Most of the time the bottom line of this is emotional appeal. That is how well someone can relate to that character either positively or negatively. We hated Joffrey because he reminded us of the spoiled little brat that lived down the street from us as a kid. We love Daenerys because she has a strong, motherly vibe to her and she's awesome.
So think about your target audience, think about the characters and their personalities. You said that they are each already significantly different from each other. They may already be memorable but because you have spent so much time working with them, they don't feel "unique" to you. My suggestion would be to give it to someone with fresh eyes and preferably void of any or most knowledge of your story. Let them experience it just like someone who were to pick up your book from the shelf without knowing anything about the book. See what they say and feel about the character development after they are done. Hopefully they give you a good honest opinion.