In my opinion the first part of the tournament is more difficult. Mainly for two reasons:
1- Lots of maniacal players who don't know what they're doing there.
2- Small value pots that don't give interesting equity.
For me if it's KO the hardest part is just until I eliminate the first player then the game appears.
If it's regular for me, the hardest thing is the beginning because usually players don't respect raises because there are a lot of blinds. In the middle and late stages players are more cautious.
Around the bubble can be very hard, you want so much to get to the money, but there are so many chips to win because everyone else wants the same.
Its very hard for me to find balance between greed and correct level of risk.
Reading through this thread I noticed that most people say that the bubble is the hardest part of the tournament and I feel the opposite way.
Seeing that only reminds me that bubble play is one of the best times to stack chips as long as you already have a dominant one.
In fact, for me the hardest part of a tournament has always been at the very end: heads-up. Nothing hurts worse than 2nd place and when you go heads up, you know your opponent is going to be tough. Bad beats sting more in heads-up and the stakes are the highest.
For me, the most difficult thing is the final stage, since the collection bubble is broken until the FT, without a doubt I should change the strategies in that instance, but it is very difficult for me