Best CV ever? No.
Best Classicvania? Yes.
It nailed everything that you want in a CV. It had great area design which was enhanced by the music. Other games like SotN and LoI had great music too, but they didn't enhance the atmosphere like in SCV4 (exception being the Ghostly Theater in LoI). Not to mention that the Treasury level was pure eye candy (the music in the area is the only one that I didn't like and failed to enhance the atmosphere of that area).
In terms of difficultly, the game started out easy and gradually got harder in a logical progression. That is until you reach the actual castle where the enemies get stronger, which is understandable. When I first got to stage 6 (the first stage that is actually inside the castle), I noticed a significant spike in difficulty. It's not as difficult as CV3, but it's still rewarding the first time you beat the game. And, let's not forget that after you beat the game, you get to play through the game on a harder difficulty.
As for the whip, I have no problem with it being multi-directional. Overpowered in terms of damage? I could see that a little. It does seem to eliminate the need for most of the sub-weapons, but to abandon the concept for the other games, that should not have been done. The only mistake that was made with the whip in my opinion is that they gave it too much reach with the second power up, which doubled its range and upped the attack strength. All that really needed to be done was debuff the that second power up in future games so that the length only increased by 50% instead of 100%. Shorter reach = greater need for sub-weapons for the axe, dagger, and boomerang. The stopwatch is always useful and fire bomb (holy water) has never widely useful except on certain situations (my opinion).