Natsu Dragneel is one of the strongest young mages in the Fairy Tail series. He's made a name for himself as The Salamander, one of Fiore's strongest, and definitely one on the path of becoming both an S ranked wizard and later, a Wizard Saint.
Despite his immense strength and promise, Natsu is not yet unbeatable. In Fiore and across the sea, on the continent of Alvarez, stand many wizards who are capable of defeating him. This being, Natsu can definitely handle himself against most opposition, as this list will illustrate.
10 GILDARTS (CAN'T BEAT)
Natsu has always admired Gildarts for his incredible strength and, as he's grown to know him more, he definitely respects him as a man. Natsu developed this habit of sneak attacking and fighting with Gildarts, and in all these events, he would lose. So, this isn't too far fetched a claim given that Natsu has never, even once, come close to beating Gildarts at anything in life. Yes, Natsu has grown exponentially but, as his last fight with Gildarts showed, he's far from the level of Gildarts. Of course, he will attain that level but, for now, he can only dream of being that overwhelmingly strong.
9 STING (CAN BEAT)
Sting Eucliffe is the current Guild Master of the Sabertooth guild. He's also one of the dragon slayers that was sent from the past, in hopes that one day he would be able to assist in bringing down Acnologia. Before his time as a guild master, Sting was most famously known for being a part of Twin Dragons of Sabertooth. This group contains Sting Eucliff, the White Dragon and Rogue Cheney, the Shadow Dragon. Natsu was capable of beating both Dragon Slayers without ever seeming to need any outside assistance. That, by itself, is more than enough proof.
8 IGNEEL (CAN'T BEAT)
Igneel was the Fire Dragon King and widely understood to be one of the most powerful dragons to ever exist throughout history. Igneel has been shown to have enough power to fight on the level of Acnologia, one of the most devastating creatures in Fairy Tail. On top of that, Igneel is the one who taught Natsu Dragon Slayer magic.
It is a hundred years too soon for Natsu to attempt that. Add to this the fact that Igneel is a legitimate, fully matured Dragon King, well versed in fire magic. Yes, Natsu is strong, but not strong enough to take on this challenge, not yet.
7 WENDY MARVELL (CAN BEAT)
Wendy is a young dragon slayer that was sent into the future as a part of the plan to eventually takedown Acnologia. She is a Sky Dragon Slayer and a member of Fairy Tail, and despite her age, she's reasonably skilled and good at support magic. In her short time at the guild, she's often been involved in guild activities, which is a testament to her usefulness. But unfortunately, these abilities, though adequate, would not help her to defeat Natsu. Natsu is both physically stronger as well as a better Dragon Slayer. Natsu, at full power, would overpower Wendy very quickly.
6 MAVIS (CAN'T BEAT)
Mavis Vermillion was the co-founder of Fiore's most powerful guild, Fairy Tail, and is one of history's strongest Mages. Her body is home to one of the strongest spells known as Fairy Heart, This magic is believed to be infinite. Mavis also knows all of Fairy Tail's strongest magic by heart and can easily use any of them. It should be remembered that anytime magic of that caliber is used , the tide of battle always changes in Fairy Tail’s favor. Natsu would probably last a few minutes, but he would not be able to overcome Mavis' magic and tactical skills.
5 JACOB (CAN BEAT)
Jacob Lessio is a member of Zeref's Spriggan 12 and, as such, he stands as one of Fairy Tail’s strongest mages. His design is a reference to Jason Statham in the transporter, which means he is, at the very least, a physically interesting character. This is one of the easier ones to comment on, given that we actually saw Jacob go up against Natsu during the series.
In that fight, he did do well for a while before eventually losing out to the immense strength of Natsu’s Fire Dragon King's flame. The fight between him and Natsu was particularly interesting due to his transporting Magic.
4 ZEREF (CAN'T BEAT)
Zeref Dragneel is quite easily one of history's strongest wizards and his legacy as the Black Wizard could be remembered throughout all of Fairy Tail history. Not many mages throughout time have been able to compete with Zeref and his curse of Ankhseram. Natsu did extremely well to deal damage to Zeref in one of their matches, but that was largely due to residual magic from Igneel. When standing on his own power, Natsu much like many other magicians, is no competition for Zeref.
3 ROGUE (CAN BEAT)
Rogue Cheney is the Shadow Dragon of Sabertooth's Twin Dragons and one of Fiore's strongest Mages. Originally sent from the past to assist the other Dragon Slayers in defeating Acnologia. The strongest attack in Rogue's arsenal would be his Dragon Force or Union Raid attack with Sting. Natsu has shown that he is able to handle both of those moves without fuss. Natsu was even shown to be too strong for a future version of Rogue, who had Shadow, White, and Dragon supremacy magic. Rogue would put up some resistance against Natsu, but he would inevitably lose.
2 ACNOLOGIA (CAN'T BEAT)
Acnologia is the most overwhelmingly powerful character in the Fairy Tail series, his strength has been enough to make even dragons fearful of him. Acnologia is a dragon slayer, hell-bent on exterminating every single dragon, due to his personal vendetta against their kind. His ability to transform into a full-grown dragon grants him enough power to devastate most of the world’s strongest mages. But, even in his human form, he can pulverize the strongest of mages in one hit. Natsu, on his own, would not stand a chance against Acnologia.
1 GAJEEL (CAN BEAT)
Gajeel Redfox is the Iron Dragon Slayer and one of Fairy Tail’s strongest mages. He was strong enough that the magic council decided to recruit him once the Fairy Tail guild had disbanded. In this role, Gajeel's strength enabled him to catch many criminals that operated throughout the land of Fiore. Gajeel was once an enemy of Natsu, before becoming his rival after joining the Fairy Tail guild. Gajeel has never been miles away from Natsu when it comes to strength, but he's never really been able to beat him.