Okay, like most Generations, most of the top Threats are in OU.
Aside from that, these are some noticably powerful pokemon, in BL
And now I'll point out some exclusive big boys in the UU tier.
Spiritomb - Having no weaknesses at the time made a Great wall, like a buffed up Sableye.
Cloyster - Even without Having Shell Smash, he still had Skill Link with was an Epic pain.
Dugtrio - A Rather popular Choice Band Sweeper with Arena Trap.
Donphan - A Nice physical Tank, With Access to Ice Shard.
Ambipomb - Amazing with Technician, Always been a favorite of mine.
Rhyperior - With that Crazy Defense Stat and His monstrous 140 Attack stat, he was a beast.
Spiritomb - Special Sweepers are the best way to go here, because they don't take a Willo Wisp Drop take a Nasty plot and Sweep.
Cloyster - Easily Conquered by Thunderbolt due to Having much higher Defense than Special Defense.
Dugtrio - Donphan is actually a Great Counter with Ice Shard.
Donphan - Well, Even though he has nice Physical Defending Stat, Special Moves sweep him out.
Ambipomb - Hitmonchan With Mach Punch normally does a Pretty good job taking this guy out.
Rhyperior - Surf. Thats all you need to take him out.
I'm not gonna do individuals on all of the OU and BL pokemon, but thats where I stand.