Many years ago, I fell in love with 8466. I had'nt purchased any lego for many years but this set had everything. Working suspension, 4 wheel drive, differentials, working transmission, huge wheels and a massive collection of parts I had never seen! I immediately bought the power puller after building the Offroader and started buying used technic sets online to bolster my collection. I took a hiatus for many years after this though and I had a similar experience this year.
Recently I discovered 42009. It was released on my birthday, and I loved it instantly. Linear Actuators (something I always wanted in Lego), TONS of gears, gearboxes, lots of functions, and it was a crane (I love construction equipment). I waited a bit to purchase it as I had just bought a house and money was tight, but after buying that set, it got me back into Lego in a ridiculous way.
In the last 4 months I have bought:
42009
42008
42024
42028
42030
42006
and 2 of the 8293 PF Sets