I'm not sure if you're looking for nouns or adjectives, here. So I'll answer both sides. ( And I'm assuming you mean "memories" as in remembered episodes of one's life, not as in someone's ability to remember. But that's ambiguous in the original question.)
To my knowledge, all adjectives meaning "lasting in memory" are neutral as to whether the memory is good or bad. There is no "memorable and immemorable" that means "lasting as a good memory" and "lasting as a bad memory" (in fact, we have memorable and immemorial, meaning "lasting in memory" and "forgotten," respectively). However, you can use "memorable" or "unforgettable" in a sentence where other words and the overall tone tell us whether it was a good memory or a bad one. If you're willing to open up to adjective + noun combinations, there's another thread already for some of that: "What is a word that means unforgettable but with a negative connotation?"
If you're looking for nouns, you're still on tricky territory. The word "memory" is a noun, and you're wanting to alter it like adjectives (famous and infamous). You can't do that to nouns. Further, the base "memory" is totally neutral, so you can't just plug a "mal-" or a "bene-" to it to make it a good thing.
"Dream" and "nightmare" are not great choices unless you use a simile for the former and/or specify elsewhere that it was a "waking" or "living" nightmare. e.g. "That day started like a dream, but quickly turned into a (waking) nightmare."
You could use "reminiscence" for a good memory, but we seldom use it as a noun, and it's rarer still to use it like a countable one. Then the opposite could be "trauma" for a bad memory that left lasting psychological scars (still kind of weird to use as a countable noun, since it might be misunderstood as an injury from a physical blow).