Itachi possed the 3 Tomoe Sharingan, which can see chakra see chakra in any form.
Copy hand movements, and copy jutsus. Also can percepts fighting movements.
KA, Shisui gave Itachi KA. With this he can pretty much One-Hit anyone. He gave him it to protect Konoha, and Itachi can use it on anyone he thinks is a threat to Konoha.
Amaterasu, black flames that burn "forever" and they are as hot as the sun. If you aren't fast its a KO
Tsukuyomi, with this he can One-Hit anyone, unless sharingan user or perfect jinchuuriki, but he Stomps.
Susanoo, humanoid figure to protect himself against attacks.
Yata Mirror, blocks and negates all attacks.
Totsuka Blade, One Hit Pierce and sealed forever, Totsuka Blitz = gg
Yasaka beads, long range attack.
Izanami to the Brain, works on "lost minded" people. K0'd DSM Kabuto
Itachi's arsenal is too over powered. He has too many Over Powered Techniques. He can't lose in a 1v1.