But this list isn't for the "best". I'm inclined to think Senna did more for McLaren in his 6 years with the team than Prost did with his 6 years.
It is the list for Mclarens best drivers.....so Prost is best....
Is this the same Senna that was there for four years before demanding a race by race contract and finally leaving on a bad note for Williams and disposing of the team that gave him 3 titles like a used tissue.
Before you say Prost did the same, Prost left cause he couldnt handle the Senna favouritism in the team, Prost left Mclaren as the world champion.
As a Prost fan I'm surprised you highlighted the 6 years each bit ,but failed to notice who Alain drove for in 1980(7 years),oh and John Watson finished above him in the championship that year 
Edit : Ayrton only got beaten by a team mate once at McLaren '89,when Mansell took him out deliberatly after being black flagged and he had a perfectly good race win taken away in japan,which would of won the title.
Alain got beaten three times by team mates at McLaren though.
enough said 

if you want to turn this into a Prost vs Senna debate youll lose too easily.
Prost thrashed World champion to be Damon Hill in the same car in 1993,
Prost thrashed World champion to be Nigel Mansell in the same car in 1990
Prost thrashed World champion Keke Rosberg in the same car in 1986
Prost thrashed World champion Aytron Senna by 21 points in the same car in 1989 (oh, and considerably under-favoured too)
Prost ANNIHALATED three time world champion Lauda in 85 , 5 times more points then Lauda.
The only times Alain lost to a teammate?
First year in F1, in which he didnt participate in 3 races compared to Watsons 100 percent.
A absurd points system in 88, he ammassed plenty more points to beat Senna, but only wins were important.
by less then a point to three time world champion Lauda.
Sennas fastest lap count 1988: 3
Prosts fastest lap count 1988: 7
Sennas fastest lap count 1989: 3
Prosts fastest lap count 1989: 5
Sennas titles: Least points compared to Prost 88, punting off rivals (Prost) to win dirty wdc's '90, totally unchallenged by any other constructor in 91.....pretty poor if you ask me.
If you need a kicker (although you have been soundly ousted) Prost also thrashed Grand Prix winners Arnoux and Alesi when teammates with him.
I got more to give if you can take it? Like the fact Senna only managed to beat Berger by a point in 92....laaammmmeeeee.
Edit: Oh and i never mentioned/highligted/bolded anything to do with how long each raced at Mclaren?
Alain Prost - Lewis Hamilton - Kamui Kobayashi - Fernando Alonso - Kimi Raikkonnen - Jenson Button