Which pokemon to power up?

I would definitely go for evolving Larvitar and power uping Scizor. For now they are good counters for Latias/Latios and TTar is always good source of damage with his high stats.

You don't have enough candy to evolve the larvitar, or enough to power up the scizor so those are out. That cloyster has a less than optimal move plus isn't particularly useful. There is no reason to have a twister gyarados so unless you tm it I wouldn't. That just leaves your alakazam as the only viable option to power up that you presented.

Edit: I missed the Exeggutor. That is an option as well. I love my confusion eggy, that could be an option as well.

I'm feeling the Larvitar should be walking buddy first and that Alakazam would be a really cool choice to power up - great for Gym defense.

My eyes are on the Scizor because it will be useful in the near future though not necessarily right now. Steel is basically the only reliable counter to Gardevoir right now, though they're uncommon in gyms and a good fire-type can send them packing. It'll be decent for the Regis too but others can do its job better. Anyways, I would pinap any Scythers you may find and look for a nest.