The strongest gift shows you really know the person. Not a standard gift voucher, but something that ties in with who they are. A cooking workshop for the foodie or a weekend away for the traveller works well. For someone who's long wanted to learn something, a course is a strong alternative.
A popular option is a joint sum of money from the team. Bundled into an experience gift voucher, or spent on something specific the departing colleague would like. Combine that with a personal memory book. All colleagues contribute something: a photo with an anecdote, or a wish for the future. That book is the gift people keep for decades.