Pokémon Resources

Pokémon Champions

Regulation M-A: Eligible Mega Pokémon
Per Victory Road, Regulation M-A will consist of Pokémon in the base Paldea Dex (minus Legendary Pokémon, Sub-Legendary Pokémon, and Paradox Pokémon) with Mega Evolution enabled. This is a list of eligible Pokémon that can Mega Evolve.

Ranked Pokémon Legends: Z-A

This chart (and each team's win rate %) is also found under Précis > CV.

Misc.

New Mega Abilities
Pokémon Showdown lists (most of) the new Mega Evolutions' Abilities once Pokémon Champions drops. Obvious spoiler warning for their new abilities before their formal announcement. Assuming Pokémon Showdown's sources are correct.
Update: Screenshots from the Japanese Pokémon Twitter feeds confirm Showdown was not accurate. Post remains for transparency and to document the discrepency.

See Also (External Links)

Pokémon Team Planner
Helps me visualize teams when planning. Link takes you to the main menu to select a game; every link from there takes you directly to whichever team you've been working on. For example, here's a random team of Mega Starmie, Mega Lucario Z, and Kyogre. Also has tools to help you narrow down which Pokémon to consider (by type, by classification, etc.).