I tend to measure "best" less on skills and more on story value. I've had some legendary weaponsmiths and whatnot (who could be trained -- by year 12, my problem is keeping the workers busy to avert rust, not training their skills), but from my point of view my "best migrant" was someone who happened to be a legendary engraver. He migrated to my fort, and decorated the halls with incredible images of history far more varied than any of my other engravers ever produced. Then he died of old age. I happened to slab him, and discovered that he was the late king consort of my civilization's last Queen (a tragic woman who deserves her own tale... and he shared in their tragedy as, among other horrors not unique to her alone, he watched all their children become gloom horrors, goblin generals, and worse). His probably-welcome death to the ravages of time came just before my current (amazing) Queen, great-grand-niece of his bride, arrived to rule the new mountainhome. I like to imagine that he achieved a late life of anonymity, hiding his past from his new neighbors (despite 94 years as husband of their ruler), while inspiring them with masterwork art depicting events they never dreamed of from their sheltered fortress lives.