There's all kinds of sub-genres within the science fiction framework (hard sf, military sf, alternate history, cyberpunk, etc.). If you haven't done a lot of SF reading, I'd recommend the more fun reads to get started.
Military SF was always a favorite of mine and has a high fun-factor. The best are Ender's Game (Card), Old Man's War (Scalzi), The Forever War (Haldeman), and Starship Troopers (Heinlein).
Other recommendations are Niven & Pournelle's Lucifer's Hammer, The Mote In God's Eye, and Inferno. Also Fredrick Pohl's Gateway.
There's lots of great stuff in the Top 100 SF list already posted but many are deep reads (Stephenson, Wolfe). Some are dated but hold up quite well for what they are with some real gems scattered about (Heinlein, Asimov, Clarke, Bester, Wells).
I say try a few authors and then use Literature-Map to find others from the ones you like. There's a lot of gold to mine in the SF genre.
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