My favorite strings are Dean Markley's vintage series. Really cool strings, but they seem to go dead fast.
(For electrics). But when Guitar Center has a Ernie Ball sale with strings at $3 a set, I have to make the switch.
For my acoustic, I absolutely love the Cleartones. Very loud strings that make my Ibanez sound like a $2,000 guitar. (I played it against my friends Gibson Songwriter Deluxe. Everyone thought my guitar sounded better!)