'Grats on the nice Entei. I was in a similar position and used a CTM (got Overheat from Fire Blast). I don't really need a Fire type attacker right now and I'm low on dust, so I'm holding off on powering it up at the moment. If you have some dust to spare and don't need to use many rare candies, I say go for it.
Entei Good Luck Bad Luck, now what?
Good news, I caught a 1926 CP, 14/15/15 (98%) entei today.
Bad news, it has Fire Fang/Fire Blast.
The question: is Entei good enough to invest TMs (one of each, fast and charge would make it undisputedly better per the rankings), and power ups with the candy and stardust?
Answers
Wait til suicune is here til u decide to power it up. U might get a 100 iv. Got 4 98 iv ones and was also lucky enough to get 100iv. It's sitting at 38.5 right now. Not sure if I am gonna take it to 40 or 39 yet. This game is about fun. If u want to use it power it up or just keep it as a trophy. It's all up to you :)
You guys are really lucky, I already did 55 enteis and caught 40 of them. I have never seen a entei above 1918. I got two 1918 entei (same stats) and caught both. Worst thing is its base stat is (14A/15D/14H) - no 15 attack. Still trying for 1918+. Or an entei with 15 attack and 96iv +. I have 270 candies (without using rare candies) but waiting for good entei. And even after doing so many raids, i never got EX Passes, I did as many different gyms as possible to get ex pass. This ex pass system is senseless :(
I'd say use one of each TM to get the current best moveset but hold off on powering.
Currently there's no raid bosses where Entei comes in useful - Exeggutor is level 2 so it doesn't need to be hard countered and no one does Venusaur raids - so there is no need to power it up right now.
You can always power it up later when it has a use, it's not like there is any benefit to powering it up now unless they decide to make power ups more expensive in the future which I doubt.
With Gen 3 coming soon and potentially new movesets being added, old movesets becoming legacy, old moves that were previously suboptimal getting buffed (e.g. Confusion Exeggutor), there's no telling what could be the best move.
Like right now Fire Blast just is a weaker version of Overheat, but Niantic might make changes to Fire Blast to make it different, so that they aren't both 1-bar, slow but high damage moves.
That being said, they might not make changes with any current moves either, so if you think that no significant changes will be made, then using one of each TM on your Entei wouldn't be a bad choice, Fire Spin is undoubtedly the best Fire type quick move and TMing away Fire Blast you're guaranteed an upgrade over it.
While 2 uses of Flamethrower are 0.2s slower than 1 Fire Blast, they have the same base damage and is more flexible with the 50 energy cost, losing 0.2s of DPS is nothing, and Overheat is better than Fire Blast in every way.
Agree with almost everything andryogynous said, with a few small caveats.
Nobody is doing venusaur raids, but with Suicune coming to the US, Venusaur is a good counter. Further, Venusaur is easier to power up now (20 candy for raid vs. 3 for Raikou/Zapdos, not available right now in the US).
And yes, there is a strong possibility that there will be rebalancing. I have two good Enteis, and used a fast TM to change FF/OH to FS/OH. The other, higher CP, has FS/FB, and I'm waiting to see what shakes out. As was said, Fire Blast is just an inferior version of Overheat, 20 points fewer damage, slightly longer time to the damage window, and slightly longer cool down. Can't see that as sustainable long term, so the move should change.
Right now is a great time to stock up on TM's because the uncertain future changes that accompany Gen3 can make everything go topsy turvy.
Exeggcutor is the way to go. The raids can be soloed and are more common, the candie is much easier to come by, Exeggcutor's Psychic typing resists Extrasensory (as opposed to Venusaur's vulnerability from its Poison typing), and Exeggcutor favors DPS over bulk, which in Suicune raids is much more desirable.