Interesting question. I'd want them in this order:
1) Half-Life 3 -- the first and second Half-Life games are among the best first-person shooters ever made. The second is Valve's greatest achievement, in my mind.
2) New IP -- the developers and programmers at Valve are some of the best in the business. It would be interesting to see what new ideas and mechanics they can summon for a new franchise.
3) Left 4 Dead 3 -- as someone who played the first two games in the Left 4 Dead series for hours on end, I would enjoy a third game with a new set of characters, new weapons, and new infected monsters.
4) Portal 3 -- as much as I love Portal 2, I think the series has run its course.










