In my opinion its between the Defiant R, and Cruiser R. The Defiant would be best damage of the 3 for a tac but also the squisheist. Either cruiser would allow for more survivability. If you skill it and are patient the Dreadnought isn't too bad, but it will be difficult to make the most of its capabilities. I would generally go with the cruiser because i don't like the super twitching nature of flying escorts. If i wanted fast twitch i'd play a space flight sim or something
Technically speaking, the Dreadnaught should be a middle ground between the Defiant and Galaxy-R, it is more Offensive, and has an stronger Tactical BO layout. On the other hand, the Galaxy-R has a BO layout geared more to Engineering, and with the saucer separated it becomes more agile than the Dreadnaught. The only thing the Dreadnaught has going for it are surprise alpha strikes from Cloak.
In general the Defiant and Galaxy-R are preferred, but what the heck? Try them all out and play all of them if they appeal to you. No one said you had to choose one and only one.