'Happy memories' is best - flashbacks tend to be seen in a negative sense! Best wishes Bob

so???? we wish them new things that they we'll remember for a long time.... what is the best way to say it?

It is a little awkward worded this way. Are you after a general "New year" greeting or saying? Are the memories you refer to of 2016? Or do you wish them to, some time in the future (eg 2018) remember 2017 with good memories? May the new year, 2017, give you ... May 2017 be filled with happy memories? To say something is just a happy memory, is like saying it is over, past, done, complete. One has to be a little careful with the wording.