I (really) think that …
I believe (that) …
I’m sure that …
In my opinion / My opinion is …
I feel that …
I guess/imagine …
I have no doubt that / I’m certain that …
I strongly believe that …
I’ve never really thought about this before, but …
My personal opinion is that / Personally, my opinion is that …
To be honest / In my honest opinion, …
As far as I know, …
I’m absolutely certain that …
I’m fairly confident that …
I’m pretty sure that …
It seems to me that …
For me/ From my point of view, …
I do believe/ feel/think …
I have come to the conclusion that …
I might change my mind later, but …
I’m (absolutely) convinced that …
To my mind / To my way of thinking, …