There are many, many stories that could fill the space of #1 on my list and make for an enjoyable list of ties. One story has really pushed forward in my recent reading to join with the other #1's, and that story is Attunga - book 1 of Tales Of The Terran Diaspora by Iarwain (37 chapters and 175,313 words).
I read Attunga over three days; I couldn't put it away. The story centers on three young men named Wirrin, Thom and Calen (a true trio), who both love one another and complement each other well, and their interactions with dolphins--particularly one special dolphin named Sonic. The story takes place in a future where large populations of people live in huge space habitats, mine asteroids for their metal and water, and artificial intelligence is a reality. Bigotry and hate have not been lost in the future, folks, and these things fuel the conflict of the story with alarming realism. Iarwain didn't fail in capturing my attention in those necessary first moments of the story, and believe me, never lets up until the very end. It's an exciting, intelligently well written story combining science (easily digestible [and acceptably believable] content), humor and the inestimable human spirit. I found it impossible not to like this story, and so Attunga can be found at the forefront of my list of #1 ties.