I don't think I can give an opinion on this without sounding like an arrogant dick. It also greatly depends on whether we are only talking about legal mountain bike trails.
ya, very hard question to answer....who are you and where are you with regards to your riding ability....
me for example...I have been riding lumpy bumpy for ~10 years now...I have done it many many times....This means that riding into it, I already have the confidence that I can clear every feature on which puts me in a relaxed mental state....dont underestimate how much this helps. Plus, now after 12 years of riding rocks all the time....how hard a trail is also depends on how long it is. Lumpy is def. in the conversation, but its relatively short and it really doesnt have much uphill in it...a little, but nothing that is very long.
I can think of 2 that really stand out. Hard to clear, LONG, exhausting.
Starting at skyline Drive...going into ringwood then returning via (I think) blue to yellow (
@JimN correct me if i got that wrong)....That is brutal return and you hit it at least 2 hours into the ride...tough uphill rock gardens and its just relentless tech and climbs.
Its not legal, but after 10 years, I would say everyone and their mother has heard about it......I have to say that all in...fugawe is still one of the toughest I can think of.....hard features, stupid straight up climbs, things you can really hurt yourself on...like tricky rollers...and still the biggest roller I have ever seen in NJ albeit not the hardest....like fugawe is one of the few loops I can think of where you can pretty much lock out your fork and leave the bike in 1st gear for ~6miles. I can go ride wildcat on a rest day and pedal around easy....fugawe, I need to eat my wheaties that morning.
black magic/tiger blood are also good ones....not quite as hard IMO bc they dont have as much climbing, but great trails, super fun.
Sourlands also great, not first in my head just bc i dont live nearby so i have only ridden there like 5 times
debacle at blue moutain is another great one
Jungle to me is a different kind of hard...I cant think of any feature (outside of the tank which I suck at) that really gives me trouble, but jungle just wears on you over time....its more like a hard park rather than a hard trail imo.
Wildcat is hard, but after so many years and so many rides there I dont feel I look at is as objectively.....but the blue trail down and back up is tough...uphill on the white trail thru the rock garden and then powering up to the hawk watch....then the REALLY hard way....bat cave, up orange, then up white to the hawk watch...thats #$%^& hard.
Max...one bit of advice for getting better at riding rocks...work on your track standing...comes in really handy.