Osman I in my opinion, though its late medieval
Probably because OP specifies 900-1500.I'm surprised no one brings up Charlemagne or Clovis, they literally ruled most powerful state of medieval western Europe and done wonders for it's development.
OP specified a time period later than 900, probably for thus exact reason.I'm surprised no one brings up Charlemagne or Clovis, they literally ruled most powerful state of medieval western Europe and done wonders for it's development.
Yes, Mehmed I is arguably the monarch who defined the Ottoman State in its modern conception. He also rebuilt the empire his father, grandfather, greatgrandfather had built when it was coming tumbling down at the hands of Tamerlane. Mehmed I is also a very good candidate as well. He was well beloved by the Christian population of his lands as well. Several anti-Latin greek cities just threw open their gates when they heard he was coming without fighting in Anatolia and Epirus. According to Caroline Finkel, he liked to walk in the cities, giving whatever he could to the people who looked at him expectantly, regardless of their faith and language - though Mehmed I did know how to speak Greek and Vulgar Bulgarian alongside his native Turkish and religious Arabic and Persian according to his court members.For a great medieval Ottoman leader I would pick Mehmed I. The Sultanate could have easily fragmented permanently after Bayezid died, but Mehmed stepped up to bat and reunified the realm and then on top of that conquered chunks of Anatolia and Albania, vassalized Wallachia, and stopped Mustafa Çelebi and Sheikh Bedredden from potentially breaking up the empire again.
Osman was a great leader, but if it weren’t for Mehmed, the Ottoman state could have crumbled before it had a chance to become as successful as it did.