Personally,I think...
I'd say that...
As I see it...
I think/believe/feel that...
It seems to me that...
In my opinion...
As far as I am concerned...
The point I'm making is...
I'd like to point out that...
Although I'm not 100% sure,I still think that...
Well,I don't know,but I'm beginning to think that...
前面几种主要用于口语,后面的一般是书面语,这些都是我们口语教程上的