Here's how I look at them:
Raiders of the Lost Ark has one the best screenplays of all time. Exposition, character development, and action have never been integrated as seamlessly as in Raiders. The script is really what elevates it above the other films in the series.
Temple of Doom has a far clunkier script, but on a technical level, it's the most accomplished of the series. The production design, effects, camera work and editing are all virtuoso stuff. It's all in service of a tonally inconsistent narrative and some sexist/borderline racist characterizations, but at least they don't get in the way of things, and the set pieces are the series' most inventive.
The filmmaking is a lot less inspired in The Last Crusade, but Sean Connery and the father/son dynamic elevate the film above what otherwise is a slightly fuzzy photo-copy of Raiders.
Kingdom of the Crystal Skull falls victim to a truly terrible script. It's all story and no narrative. Much like the Star Wars prequels, there are a lot of idea crammed in there with no clear vision as to how they should flow or be presented. The showmanship on display by Ford and Spielberg manages to make the film watchable, though sadly not re-watchable.
I'd rank them 1>2=3>>>>4. I'd gladly re-buy the first 3 films on Blu-ray, especially Temple of Doom, which should look glorious in 1080p.