Ceapaim,Masaim and Is doigh lion are examples of "I think"what are their meanings in term of context?

In Munster Irish, "Is dóigh liom" is "I suppose", Measaim is "I guess or estimate". Ceapaim is actually "I decide".

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

What's the best way to say I think

To say "I think (that blah blah)" you can say measaim, ceapaim, sílim, tá mé a cheapadh, tá mé ag meas, tá mé ag déanamh, is dóigh liom... They all mean the same thing.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

In Munster they dont mean the same thing, are they the same in Connacht and Ulster?

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

I was taught (in Cork Irish, that is) that you don't really use sílim in the present tense – but you would still normally say something like do shíleas for "I thought".

Mise leis. I tend to default to is dóigh liom in the present and use measaim as an equivalent of "I reckon" and ceapaim of "I imagine". But I agree with Lughaidh that I don't see a great of difference between these when simply used to introduce an opinion.

This surprises me as I have rarely heard Munster speakers use ceapaim for I think, it virtually always means I decide in my experience.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal