I got 20+ games, so I know how you feel.
You'll get the most storywise out of KOTOR and Deus Ex, lengthwise, so plan your time for them. They both have great replay value, so if you have the time, perhaps you should start with them and then replay them once you've finished all games.
Just Cause 2 is a game purely for messing around, so play that sporadically whenever you feel like messing around. It's a game that lasts forever.
The Portal games are a wonderful experience and is best enjoyed with the right mindset. They don't take much time, but make sure you can be focused when playing them. Very little replay value though.
Haven't heard of Aquaria.
As for which take most to get into, JC2 takes nothing, followed by the Portals that take very little, followed by KOTOR and Deus Ex, that takes extremely much to get into.