How likely is Avenger Infinity will the surprise on box office?

Basically zero. Avatar benefited from a couple things that will basically make surpassing it near impossible for the foreseeable future: It came out right as 3D films were taking off, and it was the only film at the time made purely with 3D in mind. Unlike most films that were converted to 3D in post-production, Avatar was filmed in 3D. It was made to be a purely visual feast that made use of 3D film technology to create something totally new that people hadn’t experienced before. People went and watched the film multiple times simply to experience this. Now, its novelty has worn off, so it’s not really anything special like it was originally. It remained in theatres a very long time (34 weeks). Even Star Wars: The Force Awakens, another film juggernaut, only stayed 24 weeks, a full 2.5 months less than Avatar. Avatar released in December, amid basically zero competition for a very long time. Infinity War simply won’t have anywhere near the staying power that Avatar did, because Infinity War doesn’t have the same kind of novelty that Avatar carried, and it’s releasing very close to a number of other big films (Deadpool 2, Solo: A Star Wars Story) that will heavily limit its backend ticket sales. People will go to see this film once or twice, and then wait for a home release, but Avatar was a film that demanded being seen in theatres to get the full experience of it, and many people went to see it 3, 4, or even more times, something that simply won’t happen for most viewers of Infinity War, because there’s just no need.