The five man band is a common trope in fiction, one authors regularly use to draft their characters for maximum interactive potential. The characters in a five man band are the leader, the lancer, the heart (previously known as the chick), the big guy and the smart guy. The leader leads, the lancer is the main foil, the heart connects everyone emotionally, the big guy smashes physical problems and the smart guy outthinks mental problems.
Of course in practice, this can get a little messy, and sometimes characters can fill multiple roles at once, or sometimes not all of the roles are filled at all. Things vary from story to story. However, after analyzing Star Trek for a bit, I thought I'd take a peak at how classic TOS fills out the five man band. Turns out, it is significantly weighted towards Spock.
First of all, the only ones with relevant character functions in TOS are the Triumverate. There are other side characters, of course (Scotty, Uhura, Sulu, Chevov) but they aren't the main cast and they don't serve these thematic functions from episode to episode.
Now obviously, Kirk is the leader. He's the captain. It's his thing. He leads because it is his job, and he leads because it is in his character. Thematically he's the balance between Spock and McCoy, practically he literally fills the role of captain, and relationally people want to follow him because he is a natural leader.
Likewise, McCoy is obviously the heart. He's an odd heart for sure - usually heart characters are a little more… whimsical than he is, and it's very unusual for the heart to have a combative relationship with band-mates like he has with Spock. However, he is the moral compass of the group, and he's always monitoring people's emotions and trying to find resolutions to problems that don't get people hurt.
The lancer is a complicated position, as all three members of the Triumvirate qualify to some extent, but the best fitting member is Spock. The lancer traditionally is the foil to the leader, showing a reflection of their character and often serving as their second in command. Spock is structurally the second in command, of course, and he is also the main person who plays off of Kirk, the leader.
McCoy could also be considered a lancer, however, since he also provides a foil to Kirk as the emotional side of the equation. Furthermore, lancers don't have to be assigned to the leader, and Spock and McCoy almost make better lancers to each other than they do to Kirk. Of course, Spock gets the most character development and conflict in the show, which means you could also argue Kirk is a lancer for him.
In a way, all three can be considered lancers to each other, but Spock still fits the bill best.
The category of 'big guy' is a little vague as well. Technically, Spock is physically the most powerful person on the Enterprise, being stronger and tougher than any human on the ship. When they need someone physically sturdy to destroy the giant space amoeba, that's Spock. When someone needs to survive the radiation long enough to fix the warp core, that's Spock. (Unless Kirk puts on his plot armor to go through the video game level wap core in AOS, but that's another story.)
However, Spock does not problem solve like a typical big guy. He doesn't want to smash problems into submission - even when it comes to physical combat he'd rather talk his way out of the middle of a fight than hurt someone. In Bread and Circuses, he tries to talk down the gladiator trying to kill him for as long as possible, out of a desire to prevent as much violence as he can.
The person who does bulldoze through his problems is Kirk. His problem isn't throwing a punch, his problem is stopping before he kills a helpless enemy, and sometimes it's an achievement when he doesn't do so, like with the Gorn. Kirk has a huge tendency to throw himself into fistfights, even when he's hopelessly outnumbered. He's a scrapper, and he's pretty good at it, too.
So while practically Spock functions as more of a big guy whenever someone needs to be assigned to be the strong, tough one, narratively the big guy role falls to Kirk who takes the part with great enthusiasm.
Finally, the smart guy can be no one other than Spock. Being a starship, everyone on the Enterprise must be pretty good at something, but Spock is the one know knows the most about everything. Now, Kirk is the runner-up as smart guy because Kirk is the one with the most street smarts, which is an entire subdivision of smart guy. However, Spock fulfills the role of 'guy who knows stuff' so perfectly that it's difficult to give the award to anyone else.
If we analyze these results, we can, perhaps, find explanations for some aspects of the show. Of course the Triumvirate are so beloved in their relationship together - the show maxed out their lancer potential with each other. All three are so defined by each other that they're practically impossible to separate. Of course Spock is so beloved - he has part of 3 out of 5 of the roles! Of course McCoy is so sadly neglected - he's only got one category to himself, aside from the mutual lancing. Of course the Enterprise command team is such a dangerous force - both their captain and first officer are the smart and big guys!
Also, by only giving relevant thematic roles to the Triumvirate, instead of spreading them out to the more supporting gast, TOS maximizes how much we care about each of the Big Three, leaving the other supporting cast to scrape by on charisma and groundbreaking stereotype defying character work.
It is interesting to note that the fact that roles getting messy can be a very good thing. It means the characters aren't just stereotypes, that even though they fill basic thematic functions they have complex characters beyond their functional roles. They'd be so much less interesting if they fit in neat little boxes!
To compare, look at Next Generation, which took the five man band and ran with it. I can hardly think of a property that put less work into diversifying its characters from the five man band than TNG.
The leader is obviously Picard, because he is the captain.
The lancer is obviously Riker, because he is the first officer.
The big guy is obviously Worf because he's always trying to hit stuff (however unsuccessfully).
The smart guy is obviously Data because he's the one who knows everything.
The heart is obviously Troi, because they gave her literal empathic superpowers, made her the ship counselor and followed in the longstanding literary tradition of making the heart the least developed character (aren't we glad McCoy didn't turn out like that?).
There are other characters in the show as well, obviously, but it's incredible to me that they really did go with such blatant stereotypes when TOS was so… good.