Everyone's going to have their own opinions, but my $0.02US...
If you're looking for a season with the best adventures, my pick would be Season 1, Tyranny of Dragons. The party has an obvious home base in Phlan, a number of the adventures weave throughlines from previous adventures or drop hints at what's coming in future adventures, and as the first season, even the later adventures are still surprisingly newbie-friendly.
My picks for best adventures by season:
Season 1: The Scroll Thief - Starts with some slow-paced investigation, then kicks into high gear with a chase. Finishes with a nice capstone fight, and gives options for different styles of parties to deal with the key adversary.
Season 2: Eye of the Tempest - Part 1 of a huge two-part adventure set at the Monastery of the Yellow Rose, this one is mostly about getting to the Monastery, but it's a solid wilderness adventure that, again, gives parties options as to how they want to approach the journey. (Part 2 is worth running, too, especially as a follow-up to this adventure.)
Season 3: No Foolish Matter - I would have gone with Shackles of Blood, as it's an epic-feeling adventure that combines investigation and a jailbreak, but some groups will have problems with the opening "this encounter is deliberately overwhelming because the story requires you get captured" gambit. No Foolish Matter is a quick, solid, and slightly creepy adventure that gets across some of the themes of the season as a whole.
Season 4:
The Tempter - Easily one of if not the strongest of all the Barovia adventures, The Tempter calls on a classic Ravenloft trope -- the cursed house and family -- to provide a challenging and chilling evening's adventure. I'd consider its lack of hooks into Season 4 as a whole a drawback, if Season 4 were a stronger season -- as it stands, if you only feel like running one adventure in Barovia, this is the one to run.
Season 5: Bad Business in Parnast - Another solid combination of investigation and combat, it's a solid adventure on its own, and becomes stronger if you've played through other low-tier Season 5 adventures to give the PCs hooks into the townsfolk of Parnast and a reason to want to help out and keep the place a going concern.
Season 6: no opinion - I haven't run any of these adventures, and the only one I've played, The Redemption of Kelvan, was a disappointing crawl that any rational party will be wholly unable to finish in the 2 hours stated on the cover sheet. No vote here.
Season 7: no opinion - thanks to a recent decision to allow epics to be run as charity events, I've actually played the Season 7 Epic, Peril at the Port, more than all other Season 7 modules combined. As with Season 6, I wasn't terribly impressed with the Season 7 mods I did play, so I'm not going to pretend that those are the best of the lot available. I'm sure others will have opinions.
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Pauper