Useful language for letters

To introduce new points:
With regard to…
As far as…is concerned…
I would also like to mention…

To emphasize what you are saying:
Clearly,
Obviously,
Needless to say,

Satisfaction
I thoroughly enjoyed…
I was thrilled about/by…
…was extraordinary

Dissatisfaction
I am discontented about/with…
I am not satisfied about/with…
…is not what I expected…
…does not fulfil my needs…
…is too + [adjective]

Regret/apology
I regret that I…
Please accept my apology for…
I am sorry that I am unable to…
Please forgive me for…
Regrettably I cannot …

Making a request
I would be grateful if…
What I am looking for is…
I am very keen to…
I would be indebted to you if you could…

Giving thanks
I am indeed indebted to you for…
I very much appreciated your…
I am extremely grateful to you for…