Bleach is populated with powerful, black-robed Soul Reapers who escort the souls of the dead to the waiting afterlife, and they use their zanpakuto (soul swords) to slay mighty Hollows in combat. The Captains are the strongest Soul Reapers of all, and among them is a child prodigy by the name of Toshiro Hitsugaya, Captain of squad 10.
Toshiro is a master of the blade for his age, being able to strike even against foes who have practiced the sword since before he was even born. Meanwhile, his zanpakuto, Hyorinmaru, can create ice with its shikai and bankai.
10 CAN BEAT: Captain Kaname Tosen
Captain Kaname Tosen, who commands squad 9, is a powerful Soul Reaper in his own right. He actually ranks near the bottom of the captains in terms of sheer strength and technique, but he makes up for this somewhat with his fancy kido spells.
In battle, Toshiro's sword technique and shikai could give him the edge, and if the fight ended up as bankai vs bankai, Toshiro could fill the area with ice to hit Kaname, even if he cannot hear or see anything. That ice is too much for Kaname to handle.
9 CAN'T BEAT: Ulquiorra Schiffer
Ulquiorra Schiffer, the 4th Espada, is even stronger than his boss, Sosuke Aizen, realizes. Ulquiorra has very hard hierro skin, ultra-quick reflexes, powerful cero blasts, and his released state puts him over the top. He gains wings, extra speed & strength, and the ability to make destructive energy lances.
Toshiro would struggle against Ulquiorra's released form, and partway through the battle, Ulquiorra would use his incredibly destructive Cero Oscuras and his terrifying Segunda Etapa to overwhelm Toshiro with the true might of a Vasto Lorde.
8 CAN BEAT: Lieutenant Izuru Kira
Lieutenant Izuru Kira is the second in command of squad 3, ranking right below Captain Gin Ichimaru. Izuru Kira, like his friend Shuhei Hisagi, takes his duty and his power very seriously, acknowledging the grim nature of battle. He's good with kido spells, too.
Izuru's shikai, Wabisuke, can double the weight of anything it hits, and do so more than once. Toshiro, though, is far too quick for Izuru to handle, and Toshiro would avoid Wabisuke's blade and defeat Izuru with just one strike (no bankai needed).
7 CAN'T BEAT: Captain Kenpachi Zaraki
Captain Kenpachi Zaraki has a very different attitude from Izuru and Shuhei. He doesn't fear death or destruction; instead, he revels in battle, and he seeks glory & honor in a clash of warriors every chance he gets. He has the strength to back it up, too.
Kenpachi is slower than Toshiro, but his unbelievable endurance and strength will allow him to survive anything Toshiro can throw at him. If his shikai (Nozarashi) can finish off a Sternritter like Gremmy, then it can finish off Toshiro, too.
6 CAN BEAT: Renji Abarai
Renji Abarai is the Lieutenant of squad 6, serving Captain Byakuya Kuchiki faithfully. He once belonged to squad 11 and received a great deal of training during his time there, and later, he also unlocked his bankai, Hihio Zabimaru.
Prior to obtaining his "true" bankai much later on, though, Renji lacked the power to take on Toshiro and win. Even if Toshiro can't quite match Byakuya's power, Toshiro's bankai is more than a match for Renji's, and he's better with the sword, too.
5 CAN'T BEAT: Ichigo Kurosaki
The shonen protagonist of Bleach is none other than Ichigo Kurosaki, and that means he slowly but surely built up his strength so he could rival his many powerful enemies and defeat them, one by one. Gaining a melee-oriented shikai and bankai was just the start.
Toshiro could handle Ichigo's bare-bones bankai, but once Ichigo used his Hollow mask, his powers would surge beyond anything Toshiro could handle. Even if Ichigo went down, he would get revived with the power of his inner Vasto Lorde, turning the tables once again.
4 CAN BEAT: Captain Sajin Komamura
Captain Sajin Komamura is a large, wolf-headed fellow with a good heart, but despite his size and the scale of his bankai, he is probably the weakest of the thirteen Captains. His fighting style is blunt and direct, and rather slow, too.
Toshiro would have no trouble maneuvering through Sajin's defenses to slice him apart, and even if Sajin used bankai, Toshiro's own bankai is stronger and more flexible, and it would soundly crush anything that Sajin can muster in the duel.
3 CAN'T BEAT: Yoruichi Shihoin
Yoruichi Shihoin was once the Captain of squad 2, a position that her protege (Captain Soi Fon) now holds. Then and now, Yoruichi was an expert at stealth, maneuvering, and martial arts, and she even invented a kido/melee hybrid called Shunko.
Toshiro can't defend himself properly from Yoruichi's technique, since she can ambush him from an unexpected angle and kick him right into the ground. If Toshiro tries to use shikai or bankai from there, Yoruichi would simply knock him out.
2 CAN BEAT: Chad
Yasutora "Chad" Sado learned to use his Hollow-like power, and then his Fullbring, to take on his supernatural enemies and defeat them. He may be slow and blunt, but he has the strength to defeat Privaron Espadas, fracciones and Soul Reaper Lieutenants.
Captain Hitsugaya is another story. Toshiro is too quick for Chad to handle, and his icy bankai would freeze Chad solid and prevent him from moving, just like he did to Halibel Tier. Chad has no options left.
1 CAN'T BEAT: Captain Byakuya Kuchiki
To Toshiro's credit, this would be a fairly close Captain vs Captain match. Both warriors are masters of the sword and are skilled at kido, and they have large, powerful bankai that can affect a wide area. But Byakuya has a few advantages over his younger opponent.
Byakuya has much more experience than Toshiro, and for the time being, sheer talent cannot overwhelm centuries of practice and finely honed technique. It should also be mentioned that Byakuya's kido and flash step are better than Toshiro's.