Raikou's got good offensive stats, but a single EQ off anything with decent Attack will one-shot it. Entei doesn't have a huge movepool, but it does have a few good attacks and is bulkier than Raikou. Suicune's definitely the most competitive. Her bulky stats combined with Calm Mind make her a terror to take down. Unlike the other two, she can take more than one turn to set up, meaning you get a lot more out of Calm Mind. Being bulkier also means Pressure has more time to whittle down your opponent's moves.