Ramsey continued his fine start to the season by scoring Arsenal's second goal in their 2-1 victory over Marseille at the Stade Velodrome on Wednesday night.
It was his sixth goal in seven games this season, and Merson was full of praise for the 22-year-old despite him conceding a late penalty which led to Marseille's goal.
"The lad is on fire," Merson told Sky Sports. "I just hope he doesn't take it too hard giving away the penalty - he seemed upset after the game.
"He is playing as well as anybody in the country at the moment. To score six goals already this season - we're in the middle of September - for a midfield player, it is some going.
"He is one of the ones who you didn't expect to be on this kind of role. You normally talk about Wilshere and Cazorla before you talk about Aaron Ramsey.
"But he has been absolutely outstanding and not just his goals, everything about him, he covers every blade of grass."
Theo Walcott scored Arsenal's opening goal with a stunning volley after Kieran Gibbs' cross was diverted into the forward's path.
"You could not believe how difficult it is to do what Theo Walcott did," Merson said.
"That would not be far off one of the best goals he's scored in his career so far. That was such a good finish, it was unbelievable. It was a phenomenal finish and he made it look very simple."
Despite the result, Merson believes Napoli remain the Group F favourites following their victory over Borussia Dortmund in Italy.
"It is a major, major result [for Arsenal]," Merson said. "To win away in the first game of the Champions League gets them off to a flyer. It was absolutely outstanding.
"I would have to say advantage Napoli. If Arsenal can get four points out of their next two home games, then they are on the way. You would have to say they will beat Marseille at home and that would be their 10 points.
"So they have got two big games at home, two very difficult football matches. But it is a great start. What else could you ask for?"