Some Fun Commands To Blow Stuff Up

USE AT YOUR OWN RISK! THEY MAY CAUSE EXTREME LAG OR CRASHING!
so here we go
Fireball of Doom
/summon Fireball ~ ~1 ~ {ExplosionPower:50,direction:[0.0,0.0,0.0],Motion:[0.0,0.0,0.0]}
Instant-Firing Fireball of Doom (shoots downwards)
/summon Fireball ~ ~-1 ~ {ExplosionPower:50,direction:[0.0,-4.0,0.0],Motion:[0.0,-4.0,0.0]}

Feel free to mess around with the ExplosionPower variables by changing the number. Higher = more destructive.
At 25+ though, you should probably not be in survival.

Cat Bomb
/summon PrimedTnt ~ ~ ~ {Fuse:60,Riding:{id:Ozelot}}

Bat Bomb
/summon PrimedTnt ~ ~ ~ {Fuse:60,Riding:{id:Bat}}

Explode every mob but you
/execute @e[type=!Player] ~ ~ ~ /summon PrimedTnt

Rail Bomb
/summon MinecartTNT ~ ~ ~
Do this a bunch of times whilst standing still, then move a little.

Credit to @ajdude9 for the 4 above this.

TNT Nukes

Under your feet:
/fill ~-5 ~-1 ~-5 ~5 ~-11 ~5 tnt
Over your head:
/fill ~-5 ~10 ~-5 ~5 ~20 ~5 tnt


TNT Activators:
6-and-a-bit-second primed TNT
/summon PrimedTnt ~ ~ ~ {Fuse:127}

Fireball that doesn't explode too much
/summon Fireball ~ ~1 ~ {ExplosionPower:1,direction:[0.0,0.0,0.0],Motion:[0.0,0.0,0.0]}

Ender Crystal
/summon EnderCrystal ~ ~1 ~

Flame Bow
give @p minecraft:bow 1 0 {Unbreakable:1,ench:[{id:50,lvl:1},{id:51,lvl:1}]}
Unbreakability thingy credit to @The One-eyed Guy

 

You don't need to give it Unbreaking 1000- There's a special hidden thing that allows you to make unbreakable items. No Unbreaking enchantment required.
/give @p minecraft:bow 1 0 {Unbreakable:1,ench:[{id:50,lvl:1},{id:51,lvl:1}]}

 

Ty, sir
changing that now

 

/summon PrimedTnt ~ ~ ~ {Fuse:60,Riding:{id:Ozelot}}

/execute @e[type=!Player] ~ ~ ~ /summon PrimedTnt

/summon MinecartTNT ~ ~ ~
^Do this a bunch of times whilst standing still, then move a little.

 

Can't you add {Fuse:1} to make the explosion instant? Or does it already do that?

 

Another command to crash a world. Set up the fastest redstone clock you know and set command /summon Giant ~ ~ ~ {Riding:{id:EnderDragon}}
Idk if this will work...

 

The explosion is always instant if any of the following conditions is true:

• There is no fuse.
• Fuse is set to 1-5 (ok, it isn't instant, but it's so fast you can barely tell)
• Fuse is set to 128 or higher.

So with no fuse variable, it is in fact instant. With a fuse higher than 127, it is also instant. You can do {Fuse:1} if you want, tho that will delay it by a really tiny amount. ≈1 tick.

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Just an FYI:
fastest redstone clock is a command block with the command /setblock ~ ~1 ~ redstone_block 0 destroy in it. To activate it, sneak and put a redstone block ON TOP of it.

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Added, as well as a Bat Bomb to go with the Cat Bomb.