"Best" would depend when you want to go. The cheapest time, from USA (re: your profile), would be departure between Nov 1-mid Dec, and from early Jan-late March. Personally, I like Spring and Fall. Airfares aren’t quite as cheap as winter, but you’re more likely to have some nicer weather, and less-crowded than a summer visit.

Avoid the high summer fares and depart by June 15- book about a month in advance or more and be sure you get the tickets you can get a full refund on (just in case) If you go to Amsterdam be sure to travel up north and see the Frisian Islands- expecially the one called Terschelling- you take a Ferry over and upon arriving are greeted with a 400 year old lighthouse- the island is enchanting- you can bike ride thru pine forrests and end up on the north shore where you can see one of the other islands and looks towards Iceland- My 1/2 brother lives there and works on the lighthouse so I’m partial to it- visit the pubs and play pool with the locals- so fun! and safe!