February was Harley month at DC so it makes sense Joker would want his turn. DC Comics has announced that the Clown Prince of Crime is taking over the variant covers in June, right after Convergence. This means you'll have to plan with your local comic shop to try to get all the Joker covers you want.
DC has given us the exclusive reveal for the variants to DETECTIVE COMICS #41, GOTHAM BY MIDNIGHT #6, HARLEY QUINN #17, SECRET SIX #3, and SUPERMAN/WONDER WOMAN #18.
Here's a full list of the variants so you can start your planning.
ACTION COMICS #41 by DARWYN COOKE
AQUAMAN #41 by WALTER SIMONSON and LAURA MARTIN
BATGIRL #41 by RAFAEL ALBUQUERQUE
BATMAN #41 by SEAN MURPHY and MATT HOLLINGSWORTH
BATMAN/SUPERMAN #21 by JOCK
CATWOMAN #41 by JAVIER PULIDO
DEATHSTROKE #7 by LEE WEEKS and R&T HORIE
DETECTIVE COMICS #41 by PATRICK GLEASON and JOHN KALISZ
GOTHAM ACADEMY #7 by CRAIG ROUSSEAU
GOTHAM BY MIDNIGHT by JOHN VAN FLEET
GRAYSON #9 by DAVE JOHNSON
GREEN ARROW #41 by BILL SIENKIEWICZ
GREEN LANTERN #41 by BEN OLIVER
HARLEY QUINN #17 by EDUARDO RISSO
JUSTICE LEAGUE #41 by DAVID FINCH & JONATHAN GLAPION and BRAD ANDERSON
JUSTICE LEAGUE OF AMERICA #1 by HOWARD PORTER and HIFI
LOBO #7 by NEIL EDWARDS & JAY LEISTEN and JEROMY COX
NEW SUICIDE SQUAD #9 by JIM LEE and ALEX SINCLAIR
SECRET SIX #3 by DAN JURGENS & DANNY MIKI and ALEX SINCLAIR
SINESTRO #12 by JOSHUA MIDDLETON
SUPERMAN #41 by KARL KERSCHL
SUPERMAN/WONDER WOMAN #18 by CLIFF CHIANG
TEEN TITANS #9 by SCOTT McDANIEL and R&T HORIE
THE FLASH #41 by EDDY BARROWS and MARCELO MAIOLO
WONDER WOMAN #41 by BRIAN BOLLAND
We'll update with the other covers as soon as we can.
Here's more covers.