All three years have there strengths. '07 has some absolutely classic packs for me (The Police, Bowie, QotSA) plus the singles, while the covers I bought, still get played often (particularly Fortunate Son). '08 has perhaps my favourite left-field pack of all time (Mission of Burma), plus some other solid. This year has a heap of stuff I've bought and can see getting plenty of plays (Bleach Pack, White Stripes, Tom Petty etc.)
I'll give it to '07, because even though I bought more tracks this year, I like the music from that month more. So, my rankings would be '07 > '09 > '08.