Way 1 · best default
I'd say
Your honest guess or view, said calmly. Natural for estimates too.
Examples
I'd say it's about an hour's drive.
I'd say the blue one suits you better.
In a conversation
A: How long do you think the queue is?
B: I'd say twenty minutes, maybe a bit more.
Common mistake
Not: According to me, it's too expensive.
Say: I'd say it's too expensive.
"According to" is for other people or sources ("according to the BBC"), never for yourself.