As far as boss raids go I'd go for the Rhydon first. It is like the best attacker for 50% of the raids you can solo or take down with 2 people (all the fire types, jolteon, poison types)
Dragonite is also a good choice but not that useful right now. Only real use as far as raids go is Machamp, and even then it's not too good. However, if you get a decent 3+ people attacking a higher tiered boss then the dragonite will serve you better for longevity.
Gyarados I'd leave for later. The new multipliers (for super effective, stab etc) made it even worse. It is impractical for any raid if it doesn't have hydro pump.