Svalbard, or a genuinely remote East or North Greenland itinerary. Not Iceland, and not most Greenland trips.
Svalbard has a developed expedition-cruise sector and bears can occur across the archipelago, particularly where the sea ice and the seals are. It is the most straightforward answer for someone whose main goal is a bear.
Greenland depends entirely on region. East and North Greenland expeditions may encounter bears. Many South and West Greenland trips do not, and a South or West Greenland voyage should never be sold to you as a bear trip.
Iceland has no resident polar bear population. An exceptionally rare individual occasionally arrives on drifting ice from Greenland, and that is a news story, not a holiday.
Nobody can guarantee a bear anywhere. Anyone who does is either misinformed or hoping you will not notice when it does not happen.
Two things to ask before booking. First, what proportion of the itinerary is actually in bear habitat, rather than transiting to it. Second, how the operator handles the distance rules, because in Svalbard those are legal requirements: 300 metres minimum, 500 metres from 1 March through 30 June, and no disturbing, luring or pursuing.
Those distances shape the experience more than most people expect. A responsible bear sighting is usually a long way off, and your best view will come through binoculars or a long lens rather than over the ship's rail. If somebody is offering you closer than that, they are offering you something illegal.