Hello! Always glad to see another Harley Quinn fan
First of all, very good choice starting with Mad Love. I
highly recommend picking up the
Batman: Mad Love & Other Stories trade paperback (also available in a deluxe hardcover edition) which includes not only the original Mad Love story but also many other stories that involve Harley, Joker, and many other classic Batman villains. Not all of the stories are Harley-centric, but the ones that aren't are still a lot of fun.
Next, yes, I highly recommend the original Harley Quinn run, beginning with Preludes and Knock Knock Jokes! The entire run is currently out in trade paperbacks (and a couple hardcovers) and ends with volume 4: Vengeance Unleashed. Volume 4 is a different creative team so it's not quite the same as the original 3 volumes. My favorite volume from the series, by far, is volume 3: Welcome to Metropolis. Harley goes to Metropolis with Poison Ivy, runs into Superman, clones herself in Bizarro form, and even winds up in Hell! It's hysterical, the art is wonderful, and it's a lot of fun. But the entire series is great
Finally, the New 52 run. Personally, I rather enjoy the run, as do many others, but it's all a matter of preference. I
will say that a lot of the negative reviews the New 52 run gets is because it's completely different from Harley's original solo series. She's got a completely different look, she's not following the Joker to the ends of the earth, and while Harley's original series had some really dark moments, the New 52 run has, for the most part, been full of silly, lighthearted exploits with dashes of dark humor thrown in. I've enjoyed the New 52 run a lot, but many haven't. Like I said before, it's all a matter of preference.
Anyway, hope that helps!