Elementary

1. I (really) think that …
2. I believe (that) …
3. I’m sure that …
4. In my opinion / My opinion is …

Pre-intermediate

1. I agree with …
2. I feel that …
3. I guess/imagine …
4. I have no doubt that / I’m certain that …
5. I strongly believe that …
6. I’ve never really thought about this before, but …
7. My personal opinion is that / Personally, my opinion is that …
8. To be honest / In my honest opinion, …

Intermediate

1. As far as I know, …
2. I agree with the opinion of …
3. I could be wrong, but …
4. I’d definitely say that …
5. I’d guess/imagine that …
6. I’d say that …
7. I’m absolutely certain that …
8. I’m fairly confident that …
9. I’m no expert (on this), but …
10. I’m positive that …
11. I’m pretty sure that …
12. It seems to me that …
13. It’s a complicated/difficult issue, but …
14. My (point of) view (on this) is …
15. Obviously, …
16. Some people may disagree with me, but …
17. This is just my opinion, but …
18. Without a doubt, …
19. You probably won’t agree, but …