I've always loved the first film - I think it's nearly a perfect popcorn film. It never lets up, is always entertaining, and is great fun for repeat viewings. Agreed that the supporting cast really helps - and despite being a complete ripoff of Indiana Jones, I like Fraser's McConnell. The DVD is also pretty good; great BTS on designing the Mummy, and the commentary is a lot of fun.
Mummy Returns has some nicely done set pieces - well, maybe two - but it's really a piece of shit. The resurrection scene and subsequent chase are, IMNSHO, extremely well done and huge fun to watch. Very pulp-y. And the Anbuis warriors are pretty cool. The kid is actually OK (better than Short Round, I'd say). But the "plot" itself sucks, and the whole reincarnation/rebirth thing is just stupid. The end sucks harder than the galactic core, and the Scorpion King looks like something from the Quake 2 cut scenes outtakes.
Despite trying twice, I've never made it past the first five minutes of the Scorpion King. Not only is it unfathomably bad, as pointed out above it's a barbarian film with no gore and no female nudity. What's the fucking point?