Uh... did you just ask a question and then followed it by answering to yourself? Cause that's a very thorough description you got there and very akin to what anyone here would reply. You just failed to draw in a clear conclusion. Based on your essay I'd say legacy Moltres with overheat as second charged is the strongest contender for "most-useful" fire type attacker at this moment if you can spend the resources. I have a maxed one and it is a beast, even gets auto selected for neutral matchups on occasion whereas none of my other fire types even get considered despite also being maxed out (that includes a full team of 6 BB&DC Charizard), unless I am running a graveyard and all of my top 100 Pokemon are fainted. Only then do I ever see Charizard get some sunlight.
And don't get me wrong, Blast Burn is great but I am not a big fan of Zard's frailty, so that takes some points off from it imo.
I have yet to get a decent Entei I would bother to max out tho, but I don't have a need for it tbh.
Fire-types on gym offence
Which is more useful for gym offence: Moltres, Entei or BB Charizard?
Against Exeggutor (who has Seed Bomb to punish Tyranitar), Sky Attack is better than Overheat. Sky Attack does better neutral damage, making Moltres a better generalist, covering the occasional Hariyama (of course, I could use Gardevoir if the gym also has dragons) and resisting Fairy/Ground attacks from Normal tanks. Unfortunately, Sky Attack is useless for Metagross (which is very common and tough, hence the demand for Fire attackers) and Steelix. Moltres thus benefits from a second charge move, at a substantial cost.
Whether Entei carries the powerful Overheat or the more practical Flamethrower, it is the slowest among the three, but has the best neutral bulk for absorbing Meteor Mash (and Fairy attacks from Normal tanks). Unlike the other two (which are also Flying), Entei is vulnerable to Earthquake from Metagross, Steelix and Normal tanks. Its resistance to Ice is less relevant as Ice is rare on gym defence (except for Lapras).
Charizard is the most fragile among the three and only one relevant charge move: Blast Burn (which, fortunately, is two-bar). Its main practical advantage is that it is not a legendary, so it does not require Rare Candy to power up (a level 40 Charizard may perform better than a level 30 Entei or Moltres) and can be used as a niche gym defender too.
There is also Heatran, which has even worse DPS and higher TDO than Entei. Blaziken will eventually get Blast Burn and be a contender.
Answers
On gym offense, DPS is more important than TDO. The order should go Moltres (best), BB Charizard, Heatran, Entei (worst).
Though, Charizard's DPS with Blast Burn is 98% of Moltres's DPS with Overheat, so it is nearly as good. Moltres is still a little bit better on DPS.
Entei and Heatran are nearly equally worst. Heatran has WAY higher TDO, projecting its DPS to around Entei's level. But, Heatran has awful moves, and though its attack stat is exactly the same (251), it still falls short due to bad moves in Fire Blast (rather than Overheat). Heatran makes up for this by doubly-resisting everything Fire hits for effective damage, including Fairy-types. It also resists Psychic, meaning it resists all of Blissey's moves.
Entei is the opposite, having optimal moves, but an attack stat 16 points less than Heatran and Moltres. But it keeps even with Heatran by having access to Flamethrower, the best Fire-type move for attacking gyms.
This all comes down to player preference. For maxed Fire-type DPS, use either Moltres or BB Charizard. For a Pokemon with access to moves with better parameters, go Entei, but it trades a bit of DPS along the way. For a much tankier option with double resistance for everything Fire hits effectively and resisting Blissey's entire movepool, use Heatran, but it also trades a bit of DPS. This is up to you.
Like Bellick said, you kind of answered it yourself. I'd rather go with BB Charizard or FT Entei due to reasons you listed, namely Metagross. I would not worry about EQ. It's stupidly slow and easy to dodge and Metagross especially tends to carry MM which it will almost always use before EQ has charged.
Heatran is abysmal compared to its competition, low DPS and limited to a 1-bar move. Not a contender until it gets better moves.
"Charizard is the most fragile among the three and only one relevant charge move: Blast Burn"
What about Dragon Claw - sure, there's no STAB, but it's fast, and hits almost everything for neutral damage, and Dragons for SE. Sure, there are other choices for hitting Dragons, but BB Charizard with DC is solid and covers a lot of types, especially given the bump for SE to 1.6x. Wish it had a bit more durability, though.
