Pet Sematary & Shazam seem like the best bets, La Llorona & Hellboy could make noise as well. it doesn't really matter though April will belong to Avengers I read somewhere tickets will be going on sale for Endgame on April 2nd
Easily Pet Sematary imo. Especially after the massive success of IT: Chapter 1 . Movie goers are hyped for more Stephen King adaptations from what I've noticed . This film will tide them over until IT Chapter 2 comes out.
No clue what Pet Sematary's budget is , but I bet its even less than IT's (which was around $30 million estimated ) . Now I'm not saying PS will be anywhere near as successful as IT but its gonna be definitely profitable and will most likely have good word of mouth (the reviews have been good so far) .
And on a side note, the Hellboy reboot looks disastrous . I predict it will bomb hard. The trailer(s) have been lackluster and cheap looking. Not to mention the jokes fall flat and its not like the Perlman / Del Toro Hellboy were that big of hits either. More of a cult following .