Zola's superb form at the age of 36, with nine goals to his name so far, has been a revelation after he had struggled to win a regular place last season. 
"I am speechless, he is a great player and human being. Last year he was suffering because I did not select him to play but he has responded in a fantastic way," said Ranieri.
Zola is not alone in having made an impact this season, with Chelsea's 10-game unbeaten run having taken them into second place in the Premiership and the quarter-finals of the Worthington Cup.
Ranieri added: "I was satisfied before but we were lacking continuity and this is now the best moment since I arrived in London.
"The team has more will and more determination than in the past. It is a united group, hungry for wins."
The Italian believes this progress is down to many factors, including his own developing grasp of English, as well as possessing a settled squad, with Enrique de Lucas his only pre-season signing.
Ranieri told Italian newspaper Gazzetta dello Sport: "Paradoxically, the difficult financial situation of the club, which meant that only de Lucas signed this summer, has helped us.
"There has not been any need to vary the tactical play and, in a good group, we have included several young, interesting players.
"Moreover, we have not had any grave injuries. I know my progress in learning English has also helped a lot, the boys understand me and it is good for all of us."