The benefits of the whip are that it has great reach, and that its the best one-handed weapon for disarming people. It's also useful for tripping. What you're going to have problems with is hurting your foes, since a whip can't hurt people with armor on, or creatures with natural armor above +2. Unless we create a new feat (Wounding Whip) to let us harm foes in armor, we're going to need to go in one of the two following directions.
1. Empowered magic whip-wielder.
2. Utility fighter.
The first character uses a magic whip with various enhancements and benefits that deal damage independent of the whip itself. At 16th level, I could easily imagine this character wielding a +1 flaming shocking icy thundering holy whip. Even if the target has armor on, a hit should still inflict 3d6 damage +2d6 if the target is evil +1d8 on a critical hit. The other option is a brilliant energy whip, but that's still not effective against creatures with natural armor. We could play to the whip's strength of reach by giving the character great cleave, though the drawback is that power attack itself would be a little redundant. However, in a swarm of evil creatures with 17 hit points or less, you could possibly great cleave through everything within 15 feet. For added benefit, enlarge your character to improve his reach. The great thing is that this character can wear full plate if you want, and still use the whip as a passable weapon, though he's crippled if disarmed.
The second character would most likely be a very mobile character, specializing in moving through combat and disarming or tripping foes. Compared to the first character, he suffers the great drawback that he can't do much against monstrous foes. Put him up against a dragon, and he'll just have to cry. He is best when there's a humanoid foe that is relying on a key weapon or item. Snatch away the evil high priest's holy symbol (which is possible with disarm), and you can win the day.
Now, imagine we're making a 16th level character. Can we fit these two archetypes together?