Ichigo Kurosaki
Ichigo has shown tremendous talent in raw power. What is more significant is the rate at which he gains power. The difference in power of Ichigo pre-Soul Society and now is tremendous. For what we could guess is that it has been close to one year since the beginning of the series (at least).
Shikai: There isn't much to say in this field, there is nothing really significant for Ichigo. In fact, I would say Ichigo's actual Bankai is his Shikai and Ichigo's Hollow transformation in bankai is his actual Bankai. Even without his Hollow powers, he could probably fight some of the non-Espada Arrancar.
Bankai: Ichigo's Bankai is a unique one, as said by Byakuya, all Ichigo's raw power is condensed into a very small form. This gives him 5 times more speed and double his power. His only signature attack is the Getsuga Tenshou (SP?). Even in Bankai, Ichigo has not enough strength to even match Grimmjow.
Vaizard: Despite only 11 seconds long, Ichigo's Hollow transformation gives him enough power to walk all over the 6th Espada. Depending on the difference in power between Grimmjow and the 5th Espada, Ichigo could probably take him on, too. In this form, Ichigo is a lot more aggresive in his fighting style. This is proven in his point-blank Getsuga Tenshou.
Grimmjow Jaggerjack
From what we seen so far, the 6th Espada is stronger than a Captain-level Soul Reaper. During the first battle between him and Ichigo, he only used his bare fists and still trampled all over Ichigo. It is clear that he is an extremely aggresive fighter, his style reminds me of that of a boxer.
Negacion: Grimmjow's released form has not yet been seen (in the anime). It has been revealed in the manga (I do believe). In this form, he is insanely fast.
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