Right now, Daenerys seems the strongest challenger, the only surviving child of King Aerys II Targaryen and Queen Rhaella Targaryen, the former of whom sat on the Iron Throne before Robert Baratheon.
The answer to who has the strongest claim in Westeros, Jon or Daenerys, is probably neither, but we’ll get to that.
After all, Robert’s usurping of the Iron Throne from Aerys II was respected and aided by houses Stark, Arryn, Tully, Lannister (including Ser Jaime who was key) and Greyjoy.
This might be unlikely, with Jon, Daenerys and Cersei all being such formidable leaders, but someone will surely at least raise the possibility.