Defense Deoxys

It's going to take at least 4 (3 in fog).

Raid difficulty is directly proportional to the boss's defense stat, not their attack or HP. Deoxys-D has a steep 330 base defense stat, higher than any T5 raid boss we've had so far including the Regis. Pokebattler says 4 with best friends can do it using L35 Pokemon, though I'd be a lot more comfortable with a minimum of 5 unless I knew everyone I was raiding with very well.

The good thing about EX raids is that almost everyone shows up at hatch, so it's easy to break up into groups of 6-8 so of the same team and take it down. I wouldn't worry too much about getting every possible extra ball - the base catch rate for EX raids is 6%, not the hard 2% for regular T5 raids, so unless you're throwing badly, the catch is highly likely, and if you're confident in your catching ability, silver pinaps are in order for the first several balls.