The 30-year-old is enjoying one of his finest campaigns in senior football, having featured regularly for the Citizens in their quest for a treble.
Otamendi was on the end of a 6-1 thrashing on Tuesday evening, however, as Argentina were completely taken apart by Spain at the Wanda Metropolitano.
Despite struggling to contain Spain's attackers, the former Porto and Valencia man instead picked out Barcelona's Suarez and Lukaku of rivals Manchester United as the two players who have made him suffer most.
"There are many," he told Marca. "Luis Suarez is a striker who is constantly moving, making diagonal runs, creating space, then you always have to be prepared to run. Then Lukaku is another very strong player, technically good, quick... It's between those two."
Otamendi earned his 53rd cap for Argentina in Madrid, scoring what proved to be a consolation for his side.