VIKING/ CASTLE / POTTER === Viking was the theme that first caught my eye, and it's still one of my favourites - Fun and historical. The castle & potter sets usually form the basis of MoCs for me.
'WORLDY' sets === By these I mean sets based on travelling: Prince of persia/ Orient Expedition/ Pirates Islanders / Indiana Jones all fit into this category for me, along with Taj Mahal I guess. I love these sets as they act as a proxy for real travel and look great together.
MODULAR / CITY/ CREATOR BUILDINGS ==== For my Lego town.
TECHNICS === I like MoC-ing remote control cars, just because they're fun.
TRAINS == Recently got going with monorail and I'm already a massive fan, yet to integrate it into my Lego town. Love the newer trains, not wild on the older ones. They do tend to take over a bit though space-wise!