As is often the case with sequels, it's fun to spend time with the cast again but the story is a mess. I respect directors who have a hit but know that there's no story left and end it there. Doesn't happen often, but it should.
Raiders is an easy 10 for me. Like the original Back to the Future is essentially perfect.
Temple of Doom is at best a 6, even with Amrish Puri stealing the show and having a great time. Capshaw is also very annoying and a huge, huge step back from Marion in Raiders. Reasonable then, rather like Back to the Future 2, although at least not as terrible as Matrix 2.
Last Crusade feels like Raiders but played for laughs. It's fun and Connery is good, but Marcus and Sallah are turned into over the top comic relief and are completely wasted. It gets a 7.5 from me.
I heard a rumour that there was a fourth one too, but I've seen no evidence it exists. Or maybe I just erased it from my memory.