T1 is my fav, mainly because of Reese and the love story with Sarah. T2 was amazing and still holds up. I love the calm, slow relentlessness of the T-1000. Hard to top. Both are burned into my psyche. My mum's fridge makes a three note jingle when you open it... I think of that beautifully desolate Terminator theme every time.
DF? I can barely remember it. It was forgettable (to me) and didn't really cover any new ground apart from having a (let's face it, largely unconscious) augmented human. High hopes but didn't hit the spot for me like the first 2. That rev9 guy seems to spend all of his time running sideways which got annoying quickly.
Terminator was a true classic, a great film done on a shoestring budget. At heart it was a monster movie, but instead of some undead abomination or mysterious "slasher", it was a cyborg. . .and instead of a sleepy small town or a summer camp, it was Los Angeles.
Terminator 2 was the best action film of its time, and there's a reason it's fondly remembered almost 30 years later. It was outrageously expensive, but worth every penny, and was famously groundbreaking in special effects.
There's a reason that all the various Terminator sequels and spin-offs agree on the first two films being canonical.
Terminator 3 started to drop into self parody. The shot of Arnie putting on the silly stripper sunglasses briefly after getting his clothes made it clear they KNEW they were starting to dip into self parody. They were trying to make ever-more challenging Terminators for him to fight, so they had the now absurdly powerful T-X. Despite Reese's dialogue in the first film making it clear that Skynet had lost the war and that the Terminator time travel gambit was Skynet's last chance to win the war, Terminator 3 made it seem a lot more like they just kept sending people back in time over and over again.
To me, the best part of Terminator 3 was the "world building" of finding out that John Connor was "supposed" to get in touch with the military because his girlfriend/fiancee/wife's father is a USAF Lieutenant General and that's the connection he makes that has him become a leader in the resistance after the war. The film was worth it for the scenes of Skynet activating, and Connor taking command at Crystal Peak as the whole world is wondering what's going on and why they're seeing missile launches going off.
Terminator: Salvation was part of that 2000's film trend of making everything as dark and grim as possible. Grimdark Terminator without time travel. The heart transplant ending was a last-minute plot change after test audiences absolutely HATED the original ending of Connor dying in that last attack, so that was a quick re-shoot change to let Connor live but not change a lot of scenes so the film could still come out on time.
The best part of that was the scenes with seeing the resistance actually fighting, like seeing them attacking Skynet bases.
Geneysis and Dark Fate were both forgettable sensory-overload explosion-fests that seemed to be trying HARD to cash in on the Terminator name, but that's it. Killing off John Connor in BOTH films was a huge mistake.
Dark Fate's outright flop may well have finally killed the series, but it hadn't been the same since Terminator 2 and had been slowly going downhill.