I've prepared a top 3 (in no particular order):
1. The final section of chapter 14 where you must eliminate two Nazi armored vehicles and two tanks. I was really close to throwing the controller on this one as the first parts after the checkpoint (clear the house and take out the three mortar teams) were easy but the next two were so difficult! Due to there not being a checkpoint after completing the first two, I had to keep completing this brainnumbing exercise just so I could get back to the next two objectives. Finally after being seemingly shot through the walls of the house or miscellaneous other types of death I was able to get through it. There was even one time where I blew up the final tank and was killed just after that. I was so frustrated because I finally was able to beat them and in hindsight it took me like an additional 20-30 tries before I did it again. The other major two issues I had here was the use of the guns to destroy the armoured vehicles/tanks (seriously could you have made the controls any worse Treyarch?) and the amount of times my colleagues somehow got hurt enough by me for me to fail due to friendly fire. One of the times I had a direct hit on the armored vehicle, blew it up, but somehow dominated an ally crouched behind some sandbags as well. I snapped.
2. Chapter 9 in its entirety. This level must never have been tested judging by the amount of problems I had in it. The aforementioned invincible man guarding the flak gun was a major piss off as he was immune to both shots to the face and grenades from a distance. I was finally able to kill him by rushing him and shooting him again in the face at point blank range when he came around the corner. The number two major annoyance was the fact that I continuously had the glitch happen where you lay prone and the game magically moves you from behind cover and into the line of fire. This killed me a couple of times and had me badly injured a number of other times. Terrible! Finally, towards the end of the level where you must clear first the outside of the manor and then the inside. The outside was bearable but the glitches again took over for the inside. I thought I would do something smart and snipe the people inside from the side of the manor through the window. This would have been a great concept had it not been for the invincible enemies inside. Despite breaking out all of the windows with my gun, no matter how I tried I could not hurt the enemies inside but they sure as heck could hurt me. In fact they killed me twice!
3. The hill ascent through the bunker in the forest. This one took me a good long time as well as I struggled to find the perfect path through the bunker (I unfortunately did not read the FAQs on this site before attempting). I tried flanking left, I tried flanking right but I finally determined that straight up the gut was the best way to go along with a steady diet of smoke. The neverending spawning of enemies until you move forward far enough as well as your teammates constantly dying and not helping you in anyway just added fuel to the fire. I didn't find the mortar part too, too difficult but I did somehow find a way to friendly fire a teammate once with it on my first time finally getting there. Again many curses were said.
So I finally completed veteran last night and to say that it was very satisfying to beat the game in veteran is an understatement. The satisfaction was heightened by the fact that there were so many glitches and it was so hard. In celebration I threw the disk across the room and cursed at it. I feel so much better now. I showed that game! Lol.