Sci sucks in PvE? I don't see how sci captains are any worse in PvE than engineers. Sci ships maybe, but sci captains in an escort or a dps cruiser should be fine for PvE.
SNB is almost entirely irrelevant in PvE (despite being so critical in PvP). Only once you get Photonic Fleet Science becomes more fun, of course, but at those levels, regular PvP is almost in your reach.
Engineers EPS Transfer and Nadion Inversion comes far earlier than that, and gives you some damage boost.
I'm not sure which class is... "best" all classes can tank each other indefinitely if played nearly perfectly. (except for maybe a Tac/Escort vs Tac/Escort fight). Though they have their own strengths and weaknesses.
Tacs are obviously entirely damage focused, nearly every buff they have increases damage, debuffs the target so they do more damage or recharges their tactical abilities faster so they... do more damage.
Tacs can kill people with one shot if the target is unprepared and they are set up for that function.
Engineers are nearly the opposite, with most buffs being for defense, miracle worker being for when the ship is crippled but needs just a few more seconds for a heal, eps power transfer is for raising power levels to heal more (or subsequently, when used with nadion inversaion, can do a little damage).
A properly set-up engineer can keep the attention of 2 or 3 ships and heal up to 2 of his teamates at the same time. An entire team of engineer cruisers is something no one wants to see.
Science is... mostly middle ground, they have sensor scan to increase damage, photonic fleet for more damage, if slight, dampening field for a short term but incredible defense. Subnuke is a near game breaker.
Science is frustrating to play against as they're very supportive and can do every role a little bit. Subnuke makes people cry.
Emoe makes a good point: the job of every other class and ship is simply to support we Tac/Escorts so we can make the kills and win the game. Therefore we are the most important, you are all there to help us as our servants.
All other arguments are invalid.
As for "glass cannons", any Escort can be set up to be pretty tanky if played right. Sure, not as tanky as a Cruiser or Sci Ship, but good enough to stay alive in a sustained fight long enough to blow stuff up.
The only thing Emoe is mistaken about is the idea that a Tac/Escort should be able to beat any other class/ship 1v1 with the right amount of skill. Sadly, it's not true. You can have a nigh-telepathic level of ability at anticipating an enemy's healing/buffing cycle in order to exploit a gap in that cycle to deliver your burst DPS and score a kill - but it will be absolutely worthless against a fully tank/heal-specced Cruiser or Science Ship, simply because they will not have a gap in their cooldowns to exploit. Unless they make a mistake, they'll always have something to throw on themselves to either completely or partially mitigate even your most ferocious spike in order to render it non-deadly. Especially if you're fighting an Engineer/Cruiser. There's nothing you can do except hope to eventually get an extremely lucky crit at the right time - and relying on luck is not a strategy, nor a skill (sorry Emoe, much as I love your Tric Bombing).
But that's fine, that's why STO PvP is not oriented towards 1v1 (though it's fun). And that's why Sci are always valued for their ability to Subnuke a target's buffs off them and create an opening in what would be an otherwise endless, unbeatable cycle of heals.
TL;DR - Tacs are awesome, but not OP or godlike. For the love of God, we don't need any more Engineer Zombiecruisers flying around in PvP thinking they're awesome because they're flying the most easymode combo in game.
You can't really have a "best class" in PvP in this game. Your ship choice, officer choice, BO ability choice and spec will matter much more than simply which type of officer you are. A great captain is going to excel in any role. That said, you'll have an easier time surviving or annoying the hell out of people as eng or sci. As tac you will have a bit of an easier time blowing people up if you do it right.