Going by decades since I've been reading DC:
The 1960s had a lot of upbeat comics, but if we can't include the humour comics (otherwise I'd say SUGAR AND SPIKE), then I'd name WORLD'S FINEST COMICS, because it was so great to see that Superman, Batman and Robin were all friends and supported each other in one adventure together. My runner up is ADVENTURE COMICS featuring the Legion of Super-Heroes, although it had some deaths, it presented a hopeful vision of the future.
The 1970s, I really liked SHAZAM! a lot of fun art from C.C. Beck, Bob Oksner and Kurt Schaffenberger--always made me happy--but again that's humour. And let's not talk about PLOP! and its mockery of everything under the sun. Unlike a lot of other comics, THE FLASH stayed the course through the 1970s as pure super-hero adventure and no depressing drama--until they killed off Iris West Allen at which point the series took a dark turn and never got back to being the upbeat comic it once was. And if it wasn't another parody, I'd also single out Ramona Fradon's PLASTIC MAN.
The 1980s, early in the decade, the "Whatever Happened to . . . " feature in DC COMICS PRESENTS was always something to look forward to. For the brief time it existed 'MAZING MAN took me to my happy place--oops! is that a parody? At the end of the 1980s, JUSTICE LEAGUE EUROPE was great fun--yikes, I think that's another parody.
The 1990s, started out with THE FLY from the !mpact line, with Mike Parobeck art--anything with Parobeck art lifted my spirits. YOUNG HEROES IN LOVE was not long for this world, but it was nice to hang out with those characters (dangerously close to parodic, though). And at the end of the 1990s going into the 2000s was one of my favourite comics, HOURMAN--such a good time, but not above mocking the super-hero conventions--I'm detecting a pattern.
Going on with the 2000s, if I can't include the hardcovers BIZARRO COMICS (2001) and BIZARRO WORLD (2005), because they're humour, then I have to go with MILLENNIUM EDITION titles--just because having so many reprints of great comics for a full year was one of the most upbeat experiences of my comic reading life. And the Seven Soldiers titles from Grant Morrison in 2005 proved to be time well wasted.
The return of ADVENTURE COMICS from 2009 to 2011 was good while it lasted. And I very much enjoyed Greg Rucka's second run on WONDER WOMAN from 2016/2017, too bad it didn't stick.