Tonally the first 3 are very different, both from each other and from the ones starting with the McQuarrie era with MI4: Rogue Nation. The series, despite being almost 30(?!?!) years running, has a lot of call backs and references, as well as a recurring cast of sidekicks, so watching them all is probably worthwhile, though you could probably realistically start with 4 and be fine. Then again, Kittridge is in Dead Reckoning and he was in the first film so I bet those first 3 will be relevant for some easter eggs at least in the final(?) film.
3 is kind of relevant too as Michelle Monoghan’s character is significant to 4 and one of the films after 4 (not specifying as that one is somewhat of a surprise in terms of her involvement IIRC). Plus, it has Philip Seymour Hoffman as the villain, so absolutely not skippable in my book. Not to mention 3 is the first appearance of Benji (Simon Pegg) who gradually becomes Ethan’s closest friend alongside Luther over the course of the films.
Honestly, 2 is the one true filler film. IIRC, one of the characters in a later film talks about the number of women that Ethan fell in love with, and the number they gave meant either they’re ignoring 2 in terms of canon, or they just dismissed the main girl of 2 as Ethan not actually having feelings for despite his actions in 2 suggesting otherwise.
Regardless, I would say 2 is the only one that can be easily skipped in terms of the story and characters, and IMHO a new viewer wouldn’t be missing much as the first 2/3 of the film are mostly dull and only the final act is entertaining but in a nonsensical, over the top way. And really, if one wants a more consistent John Woo directed American film that is over the top and ridiculous all the way through but also entertaining the whole time, Face/Off is a far superior choice over M:I-2.