I was a big Civ II fan, but whilst I know they have their fans I was never really sucked in to Civ III or Civ IV. For me the series got overly convoluted and lost its accessibility. Civ V changed all that for me. Effortlessly balances accessibility and depth. You can jump in and learn the ropes relatively quickly, but then just get lost for countless hours in how much there is to do. I will say that I haven't played Civ VI yet, but £7 or so for one of the best strategy games of all time would be a no brainer for me if I hadn't already given it a slice of my life.
Crazy price for the insane amount of hours included (y)
An absolute classic, worth every penny. Scary to think how many hours I've spent on CIV 5 over the years..