I believe I made one of these topics when I first came here, and at that point I was very inexperienced in the wacky realm. This is always one of my favorite topics. For each series, I am going to point out five titles of which seem more difficult in high grade for one reason or another. You all are encouraged to list some toughies in your experience as well. These are roughly listed in order from toughest in the set to least tough (out of the five that I chose), but the listings are not always so concrete.
1973 Series 1 WB
This set is unique in its sheet distribution, but it ironically does not entirely correlate with toughness in high grade. here is my list:
Kook Aid
Grave Train
Band-Ache
Lavirus
Dopey Whip
1973 Series 2 WB
This set boasts an impressive list of commonly miscut and off-centered titles, so picking five is not easy. 8-Lives and Cap'n Crud are likely candidates for the top toughest in the entire original series.
8-Lives
Cap'n Crud
Botch Tape
Brittle
Ditch Masters
1973 Series 3 TB
Conversely to series 2, I consider most of this set to be relatively easy to find in high grade. This effort is exacerbated if you don't mind egregious toning on every copy. However, a few titles stand out among the rest...
Raw Leaves
Sweathard
Wheez-It
Windchester
Rice-a-Phoni
1973 Series 4
Choke Wagon
Blue Beanie
Brute 88
Windhex
Mustard Charge
1973/4 Series 5
For reference, I have the most experience hunting for high grade titles in this set - so this list may hold a bit more water than the other series.
Shot Wheels
Muleburro
Krazy Crackers
Hungry Jerk
Triks Cereal
1974 Series 6
Clammy
Bar-Kist
Mold Rush
Truant
What Man's
1974 Series 7
Like series 3, this one doesn't generally seem difficult besides a few exceptions. I think most mid-series are this way, the majority of concern lying in roller marks.
Feetena
Leek
Contrac
Hurtz Baked Bears
Sorry Wrap
1974 Series 8
Kentucky Fried Fingers
Knots
Kong Fu
Rolaches
Hardly Wrap
1974 Series 9
This series is quite tough to pick five for! "Stickers" is an honorable mention.
Jerky Fruits
Pig Pen Oil
GI Toe
Foolite
Delinquent Spinach
1974 Series 10
Greaseline
Fishey Prize
Tic Toc
Uncle Bums
Caraid
1974 Series 11
Roller marks plague this set, and majority of the latter half of the checklist is excruciatingly difficult to find in high grade. The top three are pretty much interchangeable.
Alpain
Bash
Fib Deterrent
King o' Scare
Comit
1975 Series 12
The top three are noticeably more difficult than the rest in my opinion.
Wash 'n Fly
Battle Caps
Creep
Siesta
Martian Hats
1975 Series 13 TB
Dumb and Crazy
Doomed Matches
Don't-Touch-Mee
Ape Green Beans
Ale Detergent
1975 Series 14 WB
Ain't Toothpaste
Hippy Trash Bags
Irish Ring
Polarbearoid
Rebell Jet
1975 Series 15
This series has some of the titles with the biggest diamond cut/tilt issues in the OS.
Earth Barn
Electric Slave
Fang Edward
Jerkitol
Petley
1976 Series 16
Floral
Old Grandmom
Cracked
Ram-A-Liar
Clubbed Canadian
Let me know what you guys/gals think. Also feel free to share if you have any nice examples of some of the tough titles.