TL;DR:
as of 2016, it was Russian; its dominance is drastically decreasing nowadays; a foreigner who wants to speak on the streets of Kyiv should consider historical, political, and cultural aspects for their choice of language.
Long story.
Russian was the most comprehensible language simply because almost all Ukrainian speakers are bilingual, unlike the Russian speakers.
Calculation Method : if you simply take a group of people and count languages they are able to speak, you will notice that: Nearly 100% of those who speak Ukrainian would also have a good grasp of Russian.
So every Ukrainian speaker would count for both Ukrainian + Russian columns of your survey.
A good amount of the above also count for other languages, e.g., Ukrainian+Russian+Hebrew or Ukrainian+Russian+Crimean Tatar .
While far from 100% of Russian speakers would count to other languages.
"Russian speaker" quite often means "Russian-only speaker" .
Dare to claim that I have never seen a single "Ukrainian-only speaker".
Tendency : Russian dominance is drastically decreasing since the moment of Russian invasion to Ukraine's Crimea and Donets'k/Luhans'k regions.
Even the older people (like myself) who, during the times of forced russification, were grown in a Russian-speaking environment, now attempt to speak Ukrainian in their families.
There is no opposite process.
Age : The younger the more leaning toward Ukrainian.
"
As a foreigner, what language should I choose to speak in Kyiv?" — since this is Travel.
SE, the OP may be also interested how a traveler chooses the language to use in Ukraine.
I would recommend to read Which languages to brush up on for Ukraine trip?
question and its answers.
This answer summarizes it very well:
If your goal is to make an impression on locals I would definitely go with Ukrainian .
At the moment, Russian language is associated with Russian invasion even though a lot of people still speak Russian.
For me speaking Russian in Ukraine is like going to WWII occupied France (1) and trying to speak German with French people .
I am not telling that you will get in trouble with Russian, but you could be more considerate to the local population.
Source: I'm a citizen of Ukraine, born in Kyiv.
(1) For clarity and best accuracy, consider partially-occupied France in the period between 10 May – 25 June 1940 or France just after its liberation in 1944.
travel.stackexchange.com/users/26641/bb says Curb russian Trolls
Both Ukrainian and Russian are commonly spoken in the city;approximately 75% of Kiev's population
responded "Ukrainian" to the2001 census question on their native language, roughly 25% responded"Russian".
According to a 2006 survey, Ukrainian is used at homeby 23% of Kievans, 52% use Russian and 24% switch between both.
Inthe 2003 sociological survey, when the question 'What language do youuse in everyday life?' was asked, 52% said 'mostly Russian', 32% 'bothRussian and Ukrainian in equal measure', 14% 'mostly Ukrainian', and4.3% 'exclusively Ukrainian'.
Quoted verbatim from Wikipedia .
Source 1:
What languages are spoken in Ukraine?
- Ukraine Magazine.
Source 2: Kiev:
The City, Its Inhabitants, Challenges for Today and the Future - Zerkalo Nedeli.
travel.stackexchange.com/users/41775/Revetahw says Reinstate Monica
Someone told me most people mainly use Russian in everyday life and at home (which is what's relevant to this question), and Ukrainian in official contexts.
Is this accurate?
This is fully accurate.
Being in Kiev it is extremely rare to hear Ukrainian speech, even though a lot of the signs are in the official language.
If you speak Russian it should be extremely easy to communicate.
Source: traveling to Kiev a few times this year.
travel.stackexchange.com/users/9009/JonathanReez
In Kiev and many other regions use of Ukrainian has been increasing and instruction in most schools is in Ukrainian.
The percentage of people speaking Ukrainian "on the street" is about 30% in Kiev, 40-80% in Zhytomyr, Vinnytsya, Khmelnytskyy, Chernivtsi, Mukacheve, and Uzhhorod, 5-10% in Kharkiv, Dnipropetrovsk, and Odessa, 1-5% in Crimea, Donetsk, and Lugansk, and 80-98% in Lviv, Ivano-Frankivsk, Ternopil, Khmelnytskyy, and Lutsk (these are just approximations).
Rural areas have a significantly higher concentration of Ukrainian speakers and speakers of "Surzhyk" (Ukrainian mixed with Russian), however, the most literary Ukrainian is spoken by educated individuals in the cities.
Source: http://www.tryukraine.com/info/languages.shtml
Last update Feb 2016
If you're a Russian-speaker, you might have a hard time telling when someone's speaking Ukrainian.
I listened to "Strelkov's" news conference after the shootdown of the luckless MH17 airliner and couldn't immediately work out why someone was repeating what "Strelkov" had just said.
Then the penny dropped: he was "translating" into Russian, even though none of the words changed.
travel.stackexchange.com/users/52850/MMacD
My experience in late 2017 as someone who speaks neither language: learn the basic words/phrases in both.
Start with Ukrainian and let the other person reply in their preferred language.
No one was ever offended by my choice of language.
Like people everywhere, they are happy if you make an effort in either language.
travel.stackexchange.com/users/90343/Eric R
I'm aware that there are both native Ukrainian and Russian speakers among Ukrainians, with the west being more Ukrainian-speaking and the east/south more Russian-speaking.  What about Kiev today? I suppose the vast majority is bilingual as in most of the country, but what is the main everyday language in the city?  Someone told me most people mainly use Russian in everyday life and at home (which is what's relevant to this question), and Ukrainian in official contexts. Is this accurate?
What is the main spoken language in Kiev: Ukrainian or Russian?
travel.stackexchange.com
<ukraine><languages><kiev>
travel.stackexchange.com/questions/81213
travel.stackexchange.com/users/39782/Crazydre
Russian is more common and over 52% use it most of the time, but Ukranian is becoming more common and is used in schools and official settings. To make a good impression, Ukranian might be the better of the two, but it would be good to learn some basic words/phrases in both. If you speak Russian, it is very easy to communicate in the Ukraine. However, if you want to impress the locals, maybe learn Ukrainian. Most people in Ukraine are bilingual and they seem to choose which langauge to speak depending on the context. Ukrainian is increasingly used in schools and by much of the rural population. 
Learn Ukrainian if you want to impress the locals. Learn a few basic words in both languages. The majority of people in Kiev are bilingual, they speak Russian and Ukrainian. Use of Ukrainian is increasing and it especially widespread in rural areas. It seems people chose the langauge they use depending on the context.  It is very easy to communicate if you speak Russian.