Probably the Taboo Spare. 

I see a few guys carrying the Natural Pearl for that.

This answer depends on the bowler.  A higher rev bowler (or slower speed) could make good use of the Taboo Spare or True Blood (Taboo core stronger than the First Blood core).  But if the bowler is more matched, then perhaps the Storm Natural Pearl or even the Super Natural.  And if the bowler is rev challenged or high speed, then perhaps some of the stronger urethanes or even something like the Storm Tropical Breeze.

And even after all this, can the bowler adjust his release to use a stronger ball that could provide more versatility when the lanes become really dry.

So as you can see, it all depends on the bowler.

I agree with the opinion.  I use the Taboo Spare ball on burnt up lanes.  My team mate can throw his Nexus Soild.  I am a tweeter and he is a stroker.

Mark

This answer depends on the bowler.  A higher rev bowler (or slower speed) could make good use of the Taboo Spare or True Blood (Taboo core stronger than the First Blood core).  But if the bowler is more matched, then perhaps the Storm Natural Pearl or even the Super Natural.  And if the bowler is rev challenged or high speed, then perhaps some of the stronger urethanes or even something like the Storm Tropical Breeze.

And even after all this, can the bowler adjust his release to use a stronger ball that could provide more versatility when the lanes become really dry.

So as you can see, it all depends on the bowler.

On dry or drier lanes, it's the red/silver Pearl Urethane Storm Mix for me.

I used a polished 300C as a spare ball for a while.

I am a low rev guy.  I threw the orginal Natural as my spare ball until recently.  I also used it on occasion on very tough outside patterns.  I recently switched to a blue hammer.  I think the blue hammer is slightly better as a tough pattern strike ball.  For spare shooting urethane has been better than plastic for me.  I don't think it makes much difference which urethane you use.  As a strike ball urethane works on short patterns, and over walled house shots really well.  I think straighter players can actually make use of it better than high rev guys.  So much of the high end resin stuff has such strong core shell combinations that they force low rev guys like me to cover too many boards and lose our hit.