Without delving too much into details, the essentials I can think of are:
6-8 Mewtwo with Shadow Ball
12 Machamps
12 TTar (half rock/dark)
12 Dragons (Ray/Dragonite/Palkia/Salamence)
6-8 Kyogres
6-8 Groudons
6-8 Raikous
6-8 Moltres
6-8 Metagross (with MM)
6 Mamoswines
6-8 Venasaur (with FP)
Few others to have a couple each of may be Charizard with Blast Burn, Gengar with Lick/SB, Golems, Gyarados,
You could probably get by pretty well just with that list. Covers almost all the important raid bosses. I don't veer too far off this list and only keep Pokemon storage of 900. That is high but its mostly because I have 50 Mewtwo, Kyogre, Groudon, etc for trading.