"Incredible" literally meaning "not believable"...such a phrase is a misnomer.
But if you're looking for very intense and rare damage caused by top-end tornadoes, read damage reports from some recent EF5s such as Joplin (2011), Phil Campbell/Hackleburg (2011), or Moore (1999). Some examples of odd magnitudes of damage include parking dividers impaled by wood planks in Joplin and substantial depths of soil torn from the ground by the Phil Campbell/Hackleburg tornado.
It's kind of subjective what "incredible" means. There are ways weaker tornadoes can cause some pretty incredible things as well--like the classic tractor hanging from a debarked tree. For EF5 damage what comes to my mind are:
This can be a gory subject so just to gloss over it quick...the description of injuries and fatalities tend to be very difficult to imagine. Read up on the Bridge Creek tornado or Jarrell. Check out Jason Persoff's video on youtube about Joplin.
Unassuming objects being deadly on impact: Straw being shot through telephone poles. Pieces of wood embedded into concrete, Shingles having surgical quality cutting properties.
Cars/trucks/very heavy objects displaced by many hundreds of feet, or coming up entirely missing and never found again (Presumably made unidentifiable and spread out)
Small objects landing incredible distances away, Letters/photos from Phil Campbell Alabama found in Central Tennessee for example.
Soil scoured from the ground/road beds reduced to soil.
I'm sure there's other examples. Tim Marshall is a structural engineer and damage surveyor. Name any EF5 from the last 20 years+, he's seen the damage. Follow him on facebook, check out publications he's written. I'm sure he's left gems all over the place. He focuses heavily on poorly constructed buildings or poor construction practices--which are still very educational.
I posted this on another forum awhile back in a discussion about the problems with the EF-scale as it has been applied in practice for the last few years.
"This is in jest, but perhaps we need to go back to the "Incredible damage" descriptor from the old Fujita scale. This would encompass things like the destruction of the Cactus-117 oil rig in El Reno '11, the removal of rebar-attached concrete parking stops at Joplin, the damage to the Lingerfelt Road property at Rainsville, and the trench-digging at Philadelphia, MS to name just a few off the top of my head."