Which of the 40k RPGs are best for running a game with just one Player? I know nothing of these RPGs and barely nothing of RPGs (just one D&D 3.5 session under my belt over 15 years ago) but just in concept was thinking Deathwatch (just have the player run a Kill Team with one PC being the leader) or Dark Heresy (Inquisitor main character with a his inquisitorial acolytes, etc. as minor PCs). The latter has a bonus of being relatively new too.

I'd say a Dark Heresy campaign not focused on an Inquisitorial investigation, but on the "daily life" of one of the archetypes of people you can pick as classes. You could GM a campaign about a lone Guardsman trying to survive a plague-zombie apocalypse, or an Assassin on a solo-mission, or a Cleric / Sororitas Novice standing a lone vigil on some holy site when suddenly there are intruders, or an Arbites inspector trying to solve some high-level crime ... the list goes on. You don't need a group for adventures -- if the adventure is geared towards supporting the appearance of solo protagonists.

I would say Deathwatch , with the one PC as the permanent team leader (Veteran Sergeant?) commanding 4 (possibly rotating) NPCs.

Black Crusade. The PC is Champion of a small warband (NPCs).

Or a Rogue Trader in command of her/his family's fleet?

They are all great.

As you can see from the responses they can all work well, I do like Lynata's idea of looking of just playing a single person and looking in more depth at their life (without other players vying for attention this can be done really well).

Which game you pick should depend more on what type of game you are both interested in and whether you want the character to be more powerful or more of just someone trying to survive from day to day (some games are better for more powerful/influential characters, some are better for characters for whom each day is a fight for survival and some are flexible that you can play either type depending on your preference).

I do prefer just playing one character though so you can build their history, see the world through their eyes and possibly make some tiny bit of difference. Some games give you secondary type characters anyway who play a supporting role, such as Black Crusade with minions, Only War with comrades and Rogue Trader with.... well pretty much everyone in your dynasty if you happen to be the Rogue Trader.

Yeah, Dark Heresy lends itself more to a closer look at just what constitutes a "normal" life in 40K. Not the huge heroics or in depth investigations.

Although you could probably get a pretty sweet noir style game out of an ex arbite type of guy now lone investigator.

Although you could probably get a pretty sweet noir style game out of an ex arbite type of guy now lone investigator.

Dammit ThenDoctor ... now I have to go and write a plotline for this.

The solo Assasin story line that Lynata suggested is interesting as well.

Rain pours ceaseless in this hive city. Those who can afford an umbrella are lucky, those who can't look like their tears are scaring their face.

Someone's killed a sororitas tonight. I'm going to find out who.

::: Sigh ::: I'm on it.

It doesn't help that I have been on a Dresden File's reading kick lately.

Awesome a like from Mr. Huckelbery!

Seems like it's almost as good as the Emperor himself giving a like to somebody.

It would be an awesome beginning for a solo adventure indeed.