I think Liverpool's big change in form came when Coutinho left. Not that Coutinho is a bad player, far from it, but Liverpool changed the way they set up. This is similar to how when a new manager comes, the team get a bit of a "unknown quantity" vibe from it, as people aren't sure of how to set up. They are a good team, and they will be in the CL places, but if there aren't any big changes over the summer, I can see teams being better prepared for them (and Salah). They also play a lot of stuff on the break. Teams who don't want to go toe to toe, will be hard to get behind the back of (Like there game against United).
United are grinding results out, and will most likely continue to do that. For a team not playing good football and looking vulnerable, they are still 2nd in the league, so if they kick on (Some better signings which Mourinho actually wants), i think they will be our biggest threat.
Chelsea is all if's and buts. Depends which manager they bring in, as seems like Conte is off, and they look stale with him. Still some top players in that team though.
Spurs are Spurs. Will be 3rd or 4th. Play good football and really rate their manager, but they are still weak in certain areas and the new stadium will take a bit to bed in. Also think they'll lose the odd player and replace with "One's for the future", again, losing them some momentum.
Arsenal shouldn't even be mentioned as title challengers any more. harsh, maybe, but when have they looked like challenging this season, or the last couple of seasons. Wenger is not the force he was and while the mentality of their board is to be happy to tick over, they will not break back into the top 4. I can't see many in their squad, anyone of the top 4, or even 5, would covet. They really are losing aura of a "big club".