The Paralyzed condition in 5e dnd is one of the most deadly conditions. The reason is because hits against someone who is paralyzed is guaranteed a crit
Invisible in 5e is one of the most used conditions by players. Here is how you can go invisible, and counter invisibility.
The incapacitated condition in 5e is incurable unless it is applied using another condition. But it is rarely applied by itself. Learn more about it here.
Grappled in 5e DND is a potentially lethal condition, in the right circumstances. It is otherwise not all that bad. There are plenty of counters available.
Frightened in 5e is a somewhat annoying condition to deal with, specifically for melee users. Luckily, there are quite a few counters for it.
Exhaustion in 5e is one of the most brutal conditions that are possible. It is a condition that can directly kill your character.