however he does have Manuel I to contend with: one of the greatest (if not the greatest) of the Comnene emperors.
This is an unpopular opinion, but I find Manuel to be mostly the beneficiary of sympathetic history, rather than having been actually that great of an Emperor. He was well-recorded and well-liked in the West, so the West remembered him and wrote about him for centuries to come.
His actual reign, however, was a disaster. At his ascension to the Throne of Emperors, the Empire was in recovery, slowly but surely regaining all the territory lost in 1071. A solid hold on the most populace parts of Anatolia were firmly back in hand, and the Turks were set on what seemed to be a permanent back peddle. Most importantly, the thing that MADE the Byzantine Empire the Byzantine Empire, the professional, standing army, was rebuilt and as strong as it had been since Manzikert.
But Manuel did a very good job of squandering the resources given to him. Instead of defending in the west and advancing in the east, he split his attention a little too much, advancing in both directions. He sacrificed the short border in the Balkans for territorial aggrandizement that, in the end, was fleeting and useless. He wasted money and time on a lavish life-style imported from the courts of the West and Outremer and, perhaps most importantly, he lost the rebuilt army of his father and grandfather at Myriokephalon.
He is the Alexander to John's Phillip. He had a lot of flash and pizazz, and
SEEMS to have accomplished more, but he did all he did by standing on the shoulders of giants. Then he went ahead and tripped them all over. I truly believe that, had John not died in the 1140's, but instead lived until death of natural causes, the Empire would still be with us today. More than any other one person, the spend-thrift, flashy Manuel lelft the Empire in the vulnerable position that led to the Angeloi and, ultimately, the 4th Crusade.