Thats an open ended question LOL
I guess its whatever "floats your boat"
for me its primarily tectonics including seismology and volcanics
But I am a bit of a "jack of all trades, master at none" as I do enjoy many fields of geology
for others it may be petrology, or paleontology
Geology is just such a huge subject with so many sub and sub-sub sections that you can devote yourself to just one tiny section for a lifetime of study
Dave