The 35-year-old led Porto to the Primeira Liga triumph in 2010-11 without losing a single match, and also won the Portuguese Cup and the Europa League that season.
However, Villas-Boas failed to reach the same heights with Chelsea last season and was sacked in March, with Roberto Di Matteo eventually leading the Blues to Champions League glory.
"Remaining unbeaten in the league," he said when asked what is the bigger feat - winning the Champions League or going through the season unbeaten.
"Winning any league is extremely difficult but going through it unbeaten is also extremely difficult," added Villas-Boas. "Arsenal's achievement in 2004 was remarkable and outstanding. Like Porto in 2011, it probably won't be repeated in the modern game.
"There were games where we lived on the limit and certainly Arsenal did the same. It will be something extremely difficult to do again."