My opinion based on watching 3 days of testing:
BET AGAINST HAMILTON.
BET AGAINST MERCEDES.
Prior to testing Hamilton was around -250 for the Driver's title and Mercedes were -400 for the Constructor's title. Neither should now be considered favourites.
I know, preseason testing is unreliable, teams don't show their hand, and track conditions can favour certain teams or drivers. All that said - the Mercedes is slow! It's slow in qualifying conditions, it's slow in high fuel race simulations, the back of the car is unstable in all corner speeds, it's awful in wind, and everything looks even worse on the hard tires. Oh...and the engine and gearbox are unreliable.
Mercedes might sort out a lot of the problems in a month or 2 but there looks to be too much to fix before the 1st race in Bahrain on March 28 2021. I don't expect the Mercedes to finish in the 1st 6 in that race. I'm not convinced they'll get both cars into Q3, (top 10), on Saturday qualifying.
Right now the Red Bull is the quickest car. The Alpha Tauri and McLaren are about equal for 2nd. The Alpine, (formerly Renault) is close in 4th. Mercedes might be 5th...or anywhere. Frankly they could be 2nd or 7th by Race 1. Ferrari and Alfa Romeo are close for 6th, (about .7 of a second slower than the Red Bull PLUS Verstappen is a quicker driver). The Aston Martin, (formerly Racing Point and Force India) is 8th. The Williams is a creditable 9th because it's only about 1.4 seconds behind the Red Bull. The Haas is nowhere and isn't being developed. They will at the back every race and should be +650 to score a point. The only reason not to bet against them being last in the constructor's championship is that Williams will also struggle to score a point.
The field is a lot closer together this year than it was last year. Verstappen is the fastest F1 driver and he's in the fastest car, (combined .25 to .3 seconds clear of Ricciardo in qualification), but I don't expect him to risk using slower tires in Q2. It's that tight for top 10 qualification.
Verstappen obviously looks good for the Race 1 win and the F1 title. Red Bull should also be favourites for the Constructor's title. The only issue I have with the constructor's title is Race 2 in Italy could be shut down due to the current covid19 spike, which could also affect Race 3 in Portugal. That could give Mercedes time to recover. Even so you could reasonably expect to bet Red Bull now and hedge with Mercedes after race 1. I think Ricciardo for the title and McLaren for the Constructor's title are both worth small savers. I'll post what I think the true odds are later, (because I haven't formulated them myself yet).
Anticipated Race 1 qualifying:
1) Verstappen; 2) Ricciardo; 3) Perez; 4) Gasly; 5) Norris; 6) Alonso; 7) Tsunoda; 8) Hamilton; 9) Ocon; 10) Bottas; 11) Leclerc; 12) Sainz; 13-16) either of the Alfa Romeo/Vettel/Russell group; 17) Stroll; 18) Latifi; 19) Schumacher; 20) Mazepin.
In the race i'd expect Perez to drop a place or 2, Tsunoda could make mistakes and lose places, and the Ferraris will struggle to stay top 12. I wouldn't bet on the Haas cars finishing, (2 rookies in underfunded cars) and i'd bet against all 4 of the Mercedes and Aston Martins making it to the finish. Obviously i'm not predicting ALL that. It's a broad overview.
If nothing else stay away from Hamilton and Mercedes, at least for now.
Good luck with whatever you bet.