One of the best

Rating: 10/10 (7 July 2014)

The very first thing I have to say is that I watched the dubbed version. There were no original plays in my city anytime soon, so I had to go and watch the dubbed version. At times it was surprisingly bad dubbing for a great movie like this. IMO it partially ruined the movie experience.
Despite the bad dubbing, this movie was AWESOME. The friendship between Hiccup and Toothless had become even deeper and funnier to watch. The animating had advanced a lot from its predecessor. There were so many amazing visuals and beautiful details during the movie. The flight scenes were so well made it felt as I was flying myself. One of the best 3D experiences ever.
The only thing better than the visuals was the music. It was so beautifully made it gave me goose bumps. It was so.... I... There is no describing it well enough. You just have to hear it yourself.
Only thing that got (only a little) worse from its predecessor was the storyline. It was fun and quite well thought but still a little confusing. The were missing pieces of information all over: Where did Hiccup get his flaming sword? (Coming in next season of DW Dragons, I recall) How had Stoick got so "soft"? When had Stoick and Gobber taken their new dragons? Why they brought Eret in with so small role? (he did only a little bit and still he got Stoick's dragon)
Also after Stoick was killed, Hiccup doesn't seem to be grieving his father for very long and he doesn't seem to show personal hatred against Drago. It doesn't affect his actions visibly, witch isn't normal human behavior. This was a setback from the first movie.
I think the movie was a little too focused on visuals by bringing too many new dragon species at once. The interactions of human and dragons were a little underdeveloped (excluding Hiccup, Toothless, Valka and Cloudjumper).
IMO they should have released the third season of DW Dragons before this. They would have had more time to improve this one.
It's also interesting to see what they come up with (if they even do) to fill the spot between this film and its successor that will be released in two years.
Conclusion:
What this movie loses in a plot that isn't quite as well thought as its predecessors, it makes it up in visuals and soundtrack. It is definitely the must-see-film of the year so far. I recommend this film to everyone: it makes you happy, it makes you sad and most definitely it makes you wonder why on earth can't you live in Berk and have your own Toothless.
I give it 9/10, but it might rise to 10 after seeing it again (with original soundtrack)
------ Update ------
I re-watched the film with original soundtrack and it was a lot better. I raised the rating to 10, because now I understood the plot a lot better. If you didn't like the movie on the first time, I recommend you to re-watch it. It's definitely worth it.
------ Update 2 ------
I re-watched the film, again, with the original soundtrack. I still could find new details from the background. The visuals are just insane. If you re-watch the movie, follow the background actions too. There is so much details the eye doesn't meet the first (or the second) time.