There may not be any truly essential characters, but there are truly essential skills to place well in raids. For Phase I and Phase III, u need to b able to remove debuffs: AA, EE, Rex, Lobot. Take your pick, but cleanse is necessary. TM reduction: Teebo, ES, QGJ, Rex etc. Slow: 5's, RG, Phasma. Debuffs in Phase II, hard to stick, higher potency the better.
In conclusion, although the skills r a must, the character to do it is not...that is not to say some don't do it better than others.