Nice topic with a great title, Sci-Fi might be the biggest hobby I have in my lifetime!
About 5 years ago I started calculating how many Sci-Fi books I had already read, and the number I ended up with was around 500-600 books (on Kobo alone I have around 300 books).
I mostly read when I go to bed, I do this to prevent my mind racing off. So I literally start reading until I cannot read the letters anymore, I then put the eReader down and then usually quickly fall asleep.
Due to me having read so many Sci-Fi books, at one point in time this got me worried so much that I took some action: what worried me that much was the idea that I might had read almost all of the good Sci-Fi books already. What if new good Sci-Fi books arrive too slow to keep up with my reading pace? This really worried a lot!
So I started looking to slow down my reading-speed, in the hope that I could enjoy the books I loved a little bit longer. Among other things this made me read the text upside down, with other angles and mirror text. The only thing this resulted in however, was me being able to now read text in any angle and in the mirror pretty fast, handy for escape rooms but less to slow down my reading-speed.
In the past I had a huge bookcase with shelfs full of paper Sci-Fi book I had read. Then just before second hand paper books became almost worthless, I sold them all except a few that meant a lot to me. These days I only read from my eReader, I like the light weight of it and the red backlight when I'm reading in bed.
The eReader I have is the Kobo Clara HD B/W, if I would have to buy a new one I would buy the Kobo Clara Colour, not for the colors but for the much faster hardware it has, the extra speed is not a big deal really but when you read about 2-4 hours a day you get picky, also the on/off button is in a much better spot.
And as a last sidenote, I like to read all my Sci-Fi in the English language, this since Dutch versions often include some seriously uhm... questionable translations. For the Dutch people around here, I think the word 'Ploffer' as translations for 'Blaster' comes first to mind.
When people ask me which Sci-Fi book I liked the most, I usually recommend Hyperion, since even though the story is not for every one, it has IMHO the best writing style of all of them, you can probably open almost any random page from that book and then use the proza on that page for the lyrics of your next song, a song which would probably allow you to buy the small town of Portmeirion, Wales.
Like I wrote before, out of the big paper collection I only kept a VERY small collection of books that meant something special to me, these are those books:
- The Hitchhikers Guide to the Galaxy - Douglas Adams
This book I bought in the English language when I visited the USA for the first time in 1994. It had a big impact on me as how I saw the universe, and I just love the type of humor and with that the critique it stands for.
"In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move"
"His job is not to wield power but to draw attention away from it" -On those criteria Zaphod Beeblebrox is one of the most successful Presidents the Galaxy has ever had-
- Cities in Flight - James Blish
Dystopian earth, escaping with a complete city into the universe as a flight of liberation, the bittersweet optimism and embracing the future in uncharted space, don't look back.
Lessons for life.
- Hyperion
No extra praise needed. 
- Surface Detail - Iain M. Banks
As the book begins, a war game -the "War in Heaven"- has been running for several decades. The outcome of the simulated war will determine whether societies are allowed to run artificial Hells, virtual afterlives in which the mind-states of the dead are tortured. The Culture, fiercely anti-Hell, has opted to stay out of the war while accepting the outcome as binding.
While reading the complete Culture series by Iain M. Banks, this was one of the last books in that series I read, and it completely absorbed me. Even though it can get quite raw at times, the overal story is just such a mesmerizing philosophical thought for me, hence why it ended up on my bookshelf as an ode to what this story did for me.
- The Winged Man - A. E. van Vogt
One of the first Sci-Fi books I read when I was young, and I read it many times over and over in that era. So put that in the nostalgia category, I do not dare reading it these days since I'm scared it will spoil the magical view I have of it in my head.
The story is interesting though: A current day nuclear submarine, including it's full nuclear armament, is teleported to the far future by an unknown winged man.
Now as to some other recommendations:
- A Deepness in the Sky - Vernor Vinge
@universecannon already mentioned this one, it's also in the zones of thought universe, I liked this book quite a lot more than A Fire Upon the Deep.
- Shard of Earth - Adrian Tchaikovsky
After reading the Children of Time books, I had to read this one as well. A bit different than the Children of Time books, and I still like it.
- World Walkers - Neal Asher
A book that has The Traveler in it! 
Well rounded book, very easy to read, and lovely ideas in it.
- The Uplift War - David Brin
Quite a classic and part of The Uplift Saga. Set in a future universe in which no species can reach sentience without being "uplifted" by a patron race. But the greatest mystery of all remains unsolved: who uplifted humankind?
- Life free or Die - John Ringo
One of the more military type of Sci-Fi series (hey, it's written by John Ringo after all!), the book is a real joy to read, even humorous.
- Old Man's War series - John Scalzi
Old retired people get a second chance.
- Writer Peter F. Hamilton, the following series:
- Night's Dawn
- Commonwealth Saga (start basically as a clone of 'The Mote in God's Eye' by Larry Niven and Jerry Pournelle)
- Void
I can probably go on for hours, and this is just one part of the many pages I quickly scanned in my booklist.
Kind regards,
The Traveler
but it was pretty good.