I agree with Costas the most (although to a degree with everyone).
It depends on what you want.
If you want believable width and depth of soundstage with stable positioning of sources in the soundstage, then even inexpensive loudspeakers can best the best of headphones.
Why? Because headphones always move with your head and as your hearing and vestibular senses are connected, there will be a sensory mismatch when you move your head when listening to headphones.
This mismatch is a major contributor in the 'inside head' imaging that is common to all headphones (although to a varying degree). It is a biological fact of human hearing and you cannot escape it even with the most expensive headphones, cross-feed amplifiers or anything else except head-tracking based sound source virtualisation (and even that has a significant system lag).
But, even cheap loudspeakers stay where they are when you move your head
That is why the location cues for the sounds are much more believable to human hearing.
Also, it is impossible to keep one's head completely still while listening to such a degree that the head movement do not destroy the illusion with the best headphones.
Of course, there are many more virtues that headphones do better than loudspeakers (and vice versa), so I think it's really difficult to say which one 'the best' in all situations for all people.
It's like what Costas said: what are you after?
If you are after the most detail and least distorted frequency response, then under normal real-life conditions even $200 headphones will be better than most expensive loudspeakers.
This is due to the fact that a headphone is closely coupled to your head/ear and there is very little additional reverbation / diffraction (and associated loss of detail) with headphones.
With loudspeakers you are hearing up to 90% of REFLECTED sound of the total sound power (i.e. sound that has first reflected from a room surface before it reaches you). This means that with a loudspeaker you are always listening to a loudspeaker + room combination and the room is a significant factor in how many loudspeakers sound.
I hope that illustrates some of the major differences a bit.
best regards,
Halcyon