Level 98 is one of them because you literally go blind.
A few others are:
The Limit (old version)
Level 5.1
Your Final Goodbye
Level -998
And my most hated level is:
LEVEL -222
A: It's hard to enter, B: The level itself is dangerous and C: Most of the exits lead to dangerous levels. Level 6, Level 379, Level 888, The Void and Level -120. There is one exit that leads to Level -5, which is a Class 0. That's the only redeeming factor about this level.
(i like most of the concepts of these levels, but they would be pain to be in)