H.G. Wells The Time Machine (1895)
I was a little underwhelmed by this the first time I read it although I can no longer recall quite why; possibly due to the initially surprising contrast between the vast reputation of one of the most famous science-fiction novels of all time and it being such a short and relatively quiet little book. The story will most likely be familiar to everyone reading this - a fairly straightforward spin on earlier travelogues such as Thomas More's Utopia or even Robinson Crusoe, aside from this particular terra incognita being our distant future thus affording Wells the opportunity to offer comment on his society, its notions of progress, and the increasing class divide.
The Time Machine probably doesn't really do a great deal more than just that, but crucially it did what it does before everyone else, and did it so well as to remain pretty much the perfect science-fiction novel more than a century later. Its message is as relevant today as it ever was, and the passage of time has taken nothing from the clarity of the prose, and I'd go so far as to suggest that if you don't like this one then you've got something wrong with you.