Quick thought about Ichigo's variable power level:
Soul Society arc: As he gains more experience and develops himself, his Quincy powers are raging because quincies are stronger in environments like Soul Society. At the same time, his hollow-shinigami powers continue to awake and the Quincy Zangetsu is forced to let more of his shinigami powers loose because of overwhelming opponents like Renji, Kenpaichi and Byakuya. Quincy Zangetsu also lets him have a supressed bankai which in turn makes it easier for Hollow Zangetsu to take control.
Arrancar arc: Ichigo's power was damaging his body so Quincy Zangetsu suppresses it again making him weaker. Also, Ichigo's fear of becoming a hollow breaks his resolve and makes him even more weaker. And there's also the possibility that his badge acts as a limiter (it wasn't explained very well). After fighting Hollow Zangetsu, Ichigo manages to use again some of his power through the summon of his mask so...Soul Society Bankai Ichigo = Bankai Ichigo with mask. Against Ulquiorra Quincy Zangetsu is forced to unleash Hollow Zangetsu and we get to see some of his true power. After that, his resolve breaks again and his power varies against Yami, Aizen and Gin.
Aizen arc: He puts his shit back together, gets his resolve back and uses a sync fusion of Quincy and Hollow Zangetsu to beat Aizen. Dangai Ichigo is as such, the closer he has been to use his full power. But since the core of his powers wew still Quincy, his Final Getsuga Tensho resembles and probably is, a Vollstanding fused with his hollow-shinigami powers. And that's why he lost them.
Fullbring arc: like with Ishida, his powers were only dormant and slowly begin to awake. He starts to use fullbringer powers which in turn awake the rest of his powers faster, that's why he uses getsuga tensho copycats attacks. He loses the little piece of his power that embodies fullbring but receives a big jolt of reiatsu which re-awake all of his powers. And the remnants of his fullbring still linger and make him a bit stronger so...fullbring bankai Ichigo > bankai with mask. He is not stronger than Dangai Ichigo because Quincy Zangetsu once again suppresses a bit of his power because he doesn't want him to be a soul reaper.
Quincy arc: too damaged against Juha and Juha is just overpowered...now let's see his real power with all powers at his disposal for the first time.
I always found his bankai boring, but now it looks plain badass and destruction incarnate.