...but like one of the responders, it will completely depend on your play style. Some people are better in some escorts than other people are because of the way they play, so there is no real best per se.
Class ranks are not restricted to starship classes. I have a science officer in escort, and tactical in cruiser. Generally speaking, escorts are good dps (cannons), cruiser is good for tanking (high hull and engineer BOs), and Science vessels generally have high shields and more science BOs. If you want fast leveling, I would recommend going escort until you get to Vice Admiral, then use your saved respec tokens into what you want for end game.
A Tac in a cruiser will give you a better balance but you give up a lot of maneuverability and damage potential. Think what the starting ship is like compared to your first escort. Thats the difference between cruisers and escorts movement wise. A Tac in a escort is a glass cannon. You can put out massive amounts of damage and run rings around other ships. But you're pretty fragile. You will need to run in drawn out fights,
Also depends whether you're willing to buy from C store for your account or via emblems for a specific character.
Fleet escort - max tac with a slight emphasis on engineering (lt and ens and 3 eng console slots).
Advanced escort - max tac with a slight emphasis on science (lt and ens and 3 science console slots)
Defiant retrofit - get a cloak but your eng/sci ensign slot becomes a tactical and you lose your 3rd engineerin/science slot.
MVAM advanced escort - gives you the mvam console (can be used on the original adv escort too). The only difference is you get 1 ltc science boff and a ens and lt tac (3 sci for 3 tac abilities). You get a slightly stronger science ltc ability like TSS3 instead of a tac ability like BO3.