Wiktionary
best of all
a. superlative, the greatest of something n. Something superlative.
WordNet
best of all
adv. especially fortunate; "best of all, we don't have any homework!"
Usage examples of "best of all".
Reeve helpless, Janet Speke and the Squire in impotent rage and, best of all, Barry Erskine with his helmet-brim on his nose and blood gushing onto his shoulders.
Denny had been given another doctorate on his frog research and now had a bigger laboratory and some employees, and best of all he was still unmarried.