Pre-Roman (Iron Age) British and Irish culture. Movement of Celtic, Gaulish etc...people across Europe due to Roman expansion. I just find that period of time historically and culturally interesting.
This period of history is really interesting, but so many unanswered questions and even with DNA analysis, we probably will never get full answers. Same for the early post-Roman period in Western Europe. For example, here in Britain, how did we end up with such a clear geographic split between speakers of English and speakers of Celtic languages with hardly any borrowing of vocabulary between them and yet all the archaeology and genetics seems to show virtually no change in the population, and hardly any signs of battles?
I love that within a few miles of my house, I can visit a huge iron age hill fort, bronze age barrows, roman roads and villas, an early Anglo-saxon defensive dyke and a set of Roman era burial mounds which are over 40ft high (but not built by the Romans, who didn't go in for such things), plus a long distance path first used by neolithic flint miners, and this is a relatively insignificant location. For the most part, we don't know exactly who built any of those things, or why.
My favourite period is 17th century England. We had a civil war, a plague which killed 80% of people in some locations, the great fire of London, the restoration of the monarchy after a couple of decades as a republic, a mini ice-age, the first successful English colonies in North America and the emergence of England as a world power, Newton, Robert Hooke and co. kickstarting the scientific and industrial revolutions, and all kinds of interesting religious and political movements.