Hard sci fi tends to be tech oriented. . .and usually quasi-realistic. . .soft sci fi is often just set in the future, or a galaxy far far away.
Lots of classics are Hard, like Rama, or Asimov's Robot books. . .I'd consider the foundation books more soft sci fi.
Bio of a ST is closer to soft than hard, though many people might disagree.
EE Smith, definitely hard sci fi, if dated to hell and gone.
There's a lot of people doing hard, tech oriented sci-fi these days. . .Webber, Moon, Ringo. . .
Oh, David Gerrold. . .his space opera is pretty good, but his "War of the C'torr" books are exceptional, if only for the originality of concept