If you really want to portray yourself as the Admiral, choose the ship from which your character could best command a fleet - but feel free to take a little artistic license.
Good example - during the Falklands conflict, the British Admiral commanded from the 30 year old carrier Hermes instead of the brand new carrier Invincible. Why? Hermes was a tried and tested ship, with the space for an Admirals command staff and the necessary communications systems. By comparison, Invincible was a brand new ship with virtually no operational experience and no specific space set aside for an Admirals staff - in other words, it was newer and technically more capable but in reality Hermes was the better bet as a reliable flagship.
Before the fleet even reached the Falklands the point proved true - Hermes went the full campaign without an issue (despite her engineers literally taking bets on when the old girls boilers would give out), Invincible had a major defect in the first week.
If you're looking purely at the RP aspect, think along those lines. There's room for you to choose an Excelsior over a Galaxy, or a Nebula over a Sovereign... but a Defiant R is probably pushing it. It simply doesn't offer the space for an Admiral and his staff.