Genesis Typhoon is a nice string. It is holding tension among the best I've tried, and while not the softest out there (the softest tend to lose tension fast too), it is comfortable enough that it offers a good blend of control, spin and feel in a poly.
Another good one is Polystar Strike. Great control, nice feel, durable.
Probably the best (also pricey) is Signum Pro Hyperion. Holds tension like a beast, soft on the arm, amazing control. Cost can be prohibitive though.
But in the end, I agree with UCSF that it is best to replace a poly string often for your arm health. Even the best tension holders tend to be bad for your arm after 12-20 hours of court time. Poly just loses its elasticity over time faster than other string types. Even the softest ones will come back to bite you over time. If you start playing more often, you should be restringing that much more often.
Head Pro Tour 280/630 MP, 18x20. Tour Bite 1.15/Mosquito Bite 1.16.
Sampras serve, Agassi backhand and Federer forehand. heh heh