I found a video talking about alternatives of I think, that is very important in order to give your opinion. The video is here

There are some alternatives from informal to formal:

• I’m not going to lie, blablabla
• In all honesty [Very common], blablabla
• In my view, blablabla
• If you ask me, blabla
• As far as I can tell, blabla
• To my mind, blablabla
• As far as I’m concerned, blablabla
• The way I see it/things, blablabla
• I believe blabla
• I woud say blablabla
• To me, blablabla
• From my point of view, blabla
• I reckon [The same as I think]
• I honestly believe that
• I feel that [The same as I think]