Which are the best National Parks in Croatia?

I've never been to Paklenica NP, but I have been both at Plitvice Lakes and Krka NPs last summer. You are right that they are somewhat similar, lots of water, waterfalls (you can swim at Skradinski Buk in Krka though), green woods. Upstream, there are wider lakes, and some canyons, so there are differences, but not so big. Paklenica is completely different, it's a deep canyon in dry carst, so for the change of scenery and in order to get to know the country better, I would suggest you Paklenica this time. Also, I was in Krka NP on 1st of August, and crowds were horrendous. We took the boat to Roški slap and it was less crowded there, but we paid additional boat ticket. I think Paklenica is way less popular, which is a great thing. All three national parks (and all of Croatian NPs) are beautiful and special, each in its own way. Velebit (the mountain with two national parks, including Paklenica) is really awesome. It demands respect, so it is for more serious hikers. Paklenica is also great place for free climbing.

Since you are going there during the summer, please prepare yourself well, take enough of water, good shoes, mobile phone and a hat. And don't go out of trails. You would be surprised to know how many clueless tourists our rescuing service gets out every year from Velebit or Biokovo mountains, people are hiking in flip flops, separate from the group, overestimate distances and do many foolish things.

Been to all three of them and I would recommend Plitvice and Krka in your case. Yes, they are similar in a sense that both are famous for waterfalls, but they are still different enough. It's important to note that Krka is actually a huge park with many waterfalls in different locations, it has a beautiful small island with a monastery (Visovac), there is an interesting tourist trail built just recently, there's an old Roman settlement etc.

Most of the people go just to Skradinski Buk waterfall, which is a pity. It's beautiful there, but you miss most of the park. If you have your own car and enough time to explore it, Krka is a real gem.

Regarding Paklenica - it's really nice there, but it's more of a place for hikers. If you're into mountains and nature, you will love it. Otherwise, it might not be that interesting for you. Water is usually much more interesting to people. :)

There is another nice place close to Zadar that you might like - Vrana nature park. It's a lake with many different species. It has a nice viewpoint where you can see both the lake and the sea that's just few km away.

Thanks for the quick responses, it seems like Krka is something I wouldn't want to miss. I don't know about renting a car but I would be really interested in renting a motor bike and riding around.