I'll be very excited if RB could mount a challenge from the off. Not just because it'd make the season far more interesting but also because I'm curious to see how Verstappen would handle a title challenge year. At the minute he's clearly one of the top 2 drivers on the grid but there's a big difference in pressure between being able to win a few races throughout the year and running a sustained campaign.
Yes Hamilton's had the best car for most of his time at Mercedes but I think a lot of people underestimate the pressure that comes with putting together a championship challenge through a whole season.
I say that, as always, the truth is somewhere inbetween. He had only one title before he sat in the most dominant car in the history of the sport, losing out on at least 3 titles he had the car to win (2007, 2010, 2012). Of course you can say the same for Alonso as well who, likewise, lost out on those very same titles. In Mercedes he really didn't have much to fight, and yet Rosberg put up a challenge in most years, and now Bottas (who never looked like a particularly brilliant driver even in Williams tbh) is getting his ass handed to him. In 2017 when Ferrari was close to Mercedes, Vettel put up a serious fight until late-season issues (a crash, technical troubles, etc.) hampered him. In 2018 when Ferrari was on par with Mercedes often, Vettel was in it until he didn't lose the plot, but pace-wise there was clearly somebody putting up a fight.
He's the best driver right now, I don't think there's any question about this. Vettel is nowhere near his best, Alonso retired from F1 for now, Raikkonen's peak was over a decade ago, and drivers like Max, Leclerc (or even Norris) have yet to show their true worth once they sit in a car that can truly compete for a title. But Hamilton's legacy is obviously hard to pinpoint. Because while Schumacher narrowly lost out on titles against much better cars (1997 was insane), Hamilton lost when he wasn't in the best car and dominated when he was.