Mauricio Pochettino's Spurs go to the Nou Camp in the Champions League knowing victory would seal a spot in the knockout stages of the competition.
They are level on seven points with Inter Milan, who host PSV Eindhoven, in Group B.
Alli scored his 50th Tottenham goal in Saturday's 2-0 win at Leicester and, after Spurs failed to win their opening three group games, the midfielder feels victory in Spain would be one of the squad's greatest successes.
"It would definitely be up there," he said. "They are one of the best teams in the world, they've got some great players and as a kid you dream about playing at the Nou Camp and we're going to go there and give it our best shot.
"I've had a bit of a cold but it's not too bad. A cold won't keep me out. We're ready. It's going to be another difficult game but they're the games you want to be a part of, playing one of the best teams in the world and some of the world's greatest players.
"You want to be challenging yourself and we're going there with the mentality that we can win and we have to believe. It's going to be difficult but we have to go there and give it our best."