Season 1 is a bit stiff. Jason Dawes talks about budget/evonomics. Dry. And doesn’t get better over time.
My opinion, almost nothing epic happened. S1, for many, is apparently not on all streaming platforms.

Season 2, May joined. He was stiff, but you can hear some cynicism. Hammond is trying to come out of his shell.

Season 3 is better. The Toyota Hilux makes its famous appearance. You’ll see that Hilux on a pedestal in the studio for a long time after. Other Hilux vehicles will be in future episodes.
The sense of humor from the boys is still developing.

Season 4, episode 3, the first cheap car challenge. It’s a point in time to look at, a foundation, if you will, for almost all of the great things in their future. Hammond’s quick wit, Jeremy’s sarcasm, and May’s cynicism are more pronounced.

Here’s the thing: the earlier the season, the less sarcasm/cynicism/insults/offensive statements.
But you want to watch them because later episodes, the boys refer back. Hammond’s history of crashes, May driving like an old woman, Jeremy’s unbridled connection to the gas pedal.

Earlier seasons, they are nervous about the BBC, fans, and manufacturer backlash. They’re more conservative. They are not actively willing to come out with something that could be offensive.

It took time to gain fan support and confidence… that their level of wit wasn’t going to get them cancelled.

Later seasons, when they really open up are better because they’re willing to offend everyone for almost any reason.

Warning: There is a lot of UK regional humor. Local UK politicians and celebrities and locations.
It might not make a ton of sense to an outsider, but it’s entertaining in the sense that the gloves do come off, they’re more comfortable hurling insults.
Double entendres start to creep in. You start to see more and more how quick-witted Hammond is, James’ cynicism, and Jeremy gets more outrageous.

You see a game develop… who can one-up the others with the best insults. And it’s not just against each other, it’s anyone, for any reason. Doesn’t matter if it’s the Ferrari club, or the “lowly” Morris owner.
Originally, I didn’t know what a Morris was, but you learn quickly that they were a target, and repeatedly so. A lot of comedy was made at the expense of the Morris Owners’ Club.
And mountains of comedy directed at caravans and their owners.

And once the gloves are off, they’re off until the last episode of TGT. Even today, the boys take shots over social media.

I’d say start with S3, or 4. You can always go back and watch what you missed, see how conservative they really were.

The Vietnam special is widely regarded to be the best special. People will argue for their favorite, but if you look at upvotes over the past 10 years, Vietnam is usually the runaway favorite. Also, I think it’s particularly good becasue Jeremy was truly out of his element. All of the pain and agony one could wish on one so bold.