I only have one fast TM since I didn't raid in a while. In whom should I invest it? A 90% Weavile with ice shard and foul play, a 80% BB charizard with air slash or a 80% espeon with zen headbutt and psybeam? I know that's the worst set to espeon so with confusion it can get a little better (everytime I evolve a eevee to get an espeon it gets zh and psyb and I run out of charged TM to try to get future sight)
I'd probably save it for the time being, for something more important, until you can acquire more Fast TMs.
If you have to pick today though, go with Weavile. Sounds like it will be useful with the Lake Trio on the horizon.
Charizard and Espeon I'd say are non-urgent. Air Slash has some coverage overlap with Fire Spin, and Espeon can still perform well enough with Zen Headbutt. For sure, TM these away at some point, but not while your supplies are so low.
If you're not willing to invest CTM in Espeon for Future Sight, I would recommend you to forget about that one.
Charizard won't be the top DPS anyway, but against some of its targets it really needs Fire Spin to be useful. Air Slash is practically as good as Fire Spin for fighting Bug and Grass, but not for Steel and Ice. Personally, I'm very strict about this with all of my Charizard: they're Fire only.
Weavile is the best of its both types, but they both will probably be outclassed soon. Anyway, Tyranitar is a superb alternative for Dark at the moment and you're likely to have it powered up, but there's no comparable Ice mon all round, perhaps except for Mewtwo.
EDIT: Of course, saving it like all the others say is a wise decision too. Just depends on whether you need it now or not, really.
Yes, save it. More stuff coming out in a few days. Further, there's a new tier 5 raid boss coming Tuesday at 1 PM PST (first hatches at 2 PM). If that's Palkia or Dialga, everyone and their cousin will be doing Legendary Raids, for good reason, and you should join them. That should help with your inventory of FTMs and CTMs.
The Weavile to Snarl/Foul Play is likely the best option of the bunch. It's going to have a noticeably prominent place in the Meta as long as there are Deoxys raids, and as long as there are Psychic Legendary's.
Charizard isn't a bad idea either, but there's no call for it now. Same with Esperon - it's good, but not particularly necessary.
All in all, probably best to stockpile it. But if you're set on using it, the Weavile is the best option.
