For Platinum these are my opinions.
Cerberus:
Wave 3: 2 Atlas, 2 Phantom, 2 Brutes, 2 Ravagers
Wave 5: 2 Banshees, 2 Ravagers, 2 Phantoms, 2 Centurions
Wave 8: 1 Prime, 1 Atlas, 3 Pyros, 3 Phantoms
Wave 9: 2 Atlas, 3 Dragoons, 1 Banshee, 2 Rocket Troopers
Wave 10: 2 Alas, 2 Phantoms, 2 Banshee, 2 Bombers
Objective waves can be incredibly difficult when you have targets, as Atlas can take a while to distance themselves from the rest of the mobs and that they are very tanky. Wave 7 can be a bit hard as you have a wave of all aggro units rushing you.
Collectors:
Wave 2: 2 Praetorians(Possessed), 3 Scions(Possessed), 2 Captians(Possessed), 1 Abomination(Possessed)
Wave 4: 2 Banshees, 2 Marauders, 2 Scions(Possessed), 2 Captians
Wave 6: 2 Praetorians(Possessed), 2 Abominations, 1 Atlas, 3 Combat Engineers
Wave 8: 2 Primes, 3 Pyros, 1 Praetorian(Possessed), 2 Troopers(Possessed)
Wave 10: 2 Praetorians(Possessed, 2 Troopers(Possessed), 2 Primes, 2 Banshees
Wave 11: 1 Praetorian(Possessed),2 Abominations(Possessed)2 Primes, 3 Dragoons
Wave 9 can be really difficult for power kits due to the fact that Captains spawn in possessed and deploy so many Seeker Plagues, which make dealing with the Abominations, Phantoms and, Banshee more annoying as they are constantly chasing you.
Geth:
Wave 2: 3 Primes, 2 Pyros, 2 Rocket Troopers, 1 Hunter
Wave 6: 2 Primes, 2 Bombers, 2 Brutes, 2 Ravagers
Wave 8: 2 Banshees, 1 Ravager, 3 Pyros, 2 Rocket Troopers
Wave 9: 2 Primes, 4 Rocket Troopers, 1 Banshee, 1 Phantom
Wave 10/11: 2 Primes, 2 Hunters, 2 Banshee, 2 Phantoms
Any waves with multiple Primes can be considered hard due the possibility of Primes alternating Siege Pulse, aswell as the amount of Geth Turrets and Combat Drones they can deploy
Reapers:
Wave 2: 2 Banshees, 2 Husks, 2 Marauders, 2 Ravagers
Wave 5: 2 Banshees, 2 Hunters, 2 Pyros, 2 Ravagers
Wave 8: 2 Banshees, 3 Marauders, 1 Atlas, 2 Phantoms
Wave 10/11: 2 Banshees, 2 Ravagers, 2 Primes, 2 Phantoms
Wave 4 and 7 of Reapers can be annoying but I don't consider them too hard. I personally just hate Ravager waves.
Any wave that has multiple boss units or alot of heavy support units can make it way more difficult.