In the modern era it was in a Busch Grand National Series Race:
Only 13 cars made it back to the start/finish line with 27 cars not making it back around. Over 30 cars were involved to some extent as some cars with damage continued. I think only 2 cars finished on the lead lap. Kenny Wallace finished 9th despite losing an engine and was 11 laps down. ~
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In my opinion, there are two.
In terms of appearances and magnitude, it was Geoff Bodine's 2000 Craftsman Truck Series race at Daytona. It was truly a horrible incident and it is widely regarded a miracle that he survived. I won't link the video here (I dont know Answer.com's policy on such things), but you can go to YouTube and search "Geoff Bodine Daytona Crash" and see it.
In terms of graphicness and gore, it has to be Russell Phillips' 1995 Dash Series crash at Charlotte. His car went airborne and came to rest top first on the wall/safety railing. The top of the car sheared away on impact, grinding Phillips' body against the Fencing. He didn't survive. I hate to get too graphic, but the race was cancelled due to the massive amount of cleanup needed. There's also video of that out there, but it's just too graphic and if you wanna see, you can find it yourself.
Add: 6/20/12 by Nicholas Krzanowski
Although not NASCAR, the '24 Hours of Le Mans' race of 1955 must be rated as the #1 most devastating car crash in racing history. There is plenty of footage on youtube. 83 spectators died, 1 driver (surprisingly) and many MANY injuries. And to top it off the race CONTINUED afterwards! An extremely radical tragedy that led to major safety improvements immediately following the event. True insanity.