Something needs to be said about weaker encounters here.
While it's generally true that challenging foes are the most memorable, all encounters regardless of their strength have the potential to stand out when it comes to how well they fit within the particular milieu, how they integrate with the story and make sense in why and where they're placed, and their ability to make an impression on the player once dealt with.
If there's anything I hate it's meaningless trash encounters. They're invariably on the weaker side, usually placed simply to slow you down, soak up your time and/or resources and generally be annoying.
These are however the best opportunities for a developer to put more thought into encounter design as a whole. It's easy to place ultra-powerful creatures around every corner and laugh maniacally knowing the player is going to soon hopelessly crush his forehead against his desk out of desperation. But a few well thought-out, sensible encounters with creatures who can easily be dispatched, but that may make the player reflect on his decisions to do so, or even just to admire how far he's come in his progress, add a whole new meaning to playing the game itself.
Dragonlings that have to be killed for the safety of the town. Orc tribe pups who've all been diseased. Predatory birds that are otherwise harmless interfering with the livelihood of a fishing village.
These are examples of encounters that can impact the player's impression of a game just as much as the Foozle at the end.