In Spanish an opinion rarely arrives bare. It comes with an opener, and the opener tells your listener how firmly you hold the view. From softest to firmest:

Opener	English
Creo que… / Me parece que…	I think… / It seems to me…
Yo diría que…	I'd say…
En mi opinión,…	In my opinion,…
Para mí,…	For me,… / To me,…
Desde mi punto de vista,…	From my point of view,…
Sinceramente,… / La verdad es que…	Honestly,… / The truth is…
Estoy seguro / segura de que…	I'm sure…
Sin duda,…	Without a doubt,…

• Yo diría que uses the conditional, which softens just as English I'd say does.
• Seguro agrees with the speaker — Marcos is seguro, Marina segura — and it goes with estar: certainty is a state of mind.
• Careful speakers keep the de: estoy seguro de que. It often disappears in fast speech, but in writing it belongs there.