I think I like McCoy the most of all. He's hilarious!
McCoy the character shades a great many scenarios over the run of TOS. The interaction with Spock is simply classic; their rivalry is the stuff of fine writing and mutual if subtle character development, and speaking of character, McCoy demonstrated an abundance of it very early in the series. Despite his fears and role (more towards the back of the bus as opposed to leadership), he stepped-up and gambled his life in Miri, and later in The Empath, the latter not truly being due to anything of his hand, the former an extension of his taking responsibility on something he had worked on.
And I absolutely enjoy DeForrest Kelley's portrayal of McCoy; a character that is admittedly very human, flawed, colorful and at-times, properly over-the-top.
I forgot the empath, he gets tore pretty tore up in that one. Can't forget shore leave when he "dies" being the knight in shining armor!