Definitely Splinter Cell.

When I think back to Splinter Cell: Pandora Tomorrow with these cool settings like the djungles of Indonesia, the maritime docking station for submarines, the railways in Paris, the US embassy in East Timor and the adventure in Jerusalem, I would love to see a Battle Royal mode.

The way this would look is that depending on the number of participants in the battle royal, each one will have one or more allies (so there could be something like groups of three or four as well), yet to add some spiciness into the mode, nobody will know who is allied with whom upon entering the game, for the computer randomly and secretly decides this so that each one would have to find out for themselves!

The catch here is that as a participant you don't know whom you can trust. So it can very well be that you hide from your own ally and the both of you chase each other without noticing that you are allies.

Here, another cool option comes in: the possibility of killing your ally. In case you turn this option off, this would mean that you will finally know that this is your ally because you are impervious to his guns' bullet, or when he sneaks behind you to garrote you it says: not possible. From then on, you can cooperate.
Yet in case you turn this option on, it might very well be that you kill your own ally. This option would premise that one participant knows with whom they ally in that battle. Of course, this option requires more of a sure instinct by both. The one who knows his ally would need to develop some ideas as to how he can signal this to his ally, while the ally would have to interpret the signals accordingly.

This allows for tons of unique scenarios. Then let's say that out of all those detective gadgets you can choose three and out of all guns you can choose 3 as well, allowing for some more strategic approach, even in case you and your ally choose the same gadgets and guns. Finally, you could win bonus points for certain achievements, like killing your opponent most stealthily, or cooperative combos..

Add in the great soundtrack, and fun is guaranteed! ;)