It very much depends on the occasion in which the statement will be made. If it is a very formal occasion, "we are very grateful to the late Mr./Mrs./Miss/Ms. _________…" is appropriate. If one were speaking where relatives or very close colleagues of the deceased person would be present, it might be advisable to say, "We are very grateful to ___________, whom we remember fondly, for…" I would warn against trusting ALKONAS or other dictionaries for the meaning of "fond"/"fondly" in this context. This is a very common American English statement.

In some cases, "Dear" may not be appropriate in verbal communication of this sort (though if it is a written note of thanks, I think it would certainly be okay). It may be better to say, "Honored guests, …" with proper attention to protocol if certain of those present need to be individually addressed (e.g., ambassadors, legislators, judges, etc.).