Perhaps controversially, I'd go Revenge of the Mummy here, and by quite some distance.
I've always liked ROTM; I love the theming, and I think the ride is pretty good from memory, too (although granted, I haven't ridden since 2016), with some fun launches and an exciting layout! Certainly a solid 8/10 coaster, and it sits in my top 15!
Everest is all right, but it always leaves me feeling slightly cold, and I'll admit that I don't get the hype. It's a perfectly fun family coaster, it's stunningly themed, and it has some nice tricks to it, but if I'm being nitpicky, it isn't the most dynamic in terms of transitions compared to others I've ridden, and it feels a tad stop-start, with the bit where you're stopped for ages in front of the broken track really killing the flow, in my opinion. I've always heard people wax lyrical about it, and I went in super hyped, but I was bitterly disappointed when I first rode it and even though it's a perfectly fun ride, it always leaves me feeling a tad underwhelmed. It's still a perfectly fun ride, though; a solid 6/10 for me, and sits at #40 of the 91 coasters I've ridden.
They are similar to me in the sense that they both slaughter their overall pacing in the name of gimmicks or theming sequences, which is fine I suppose. And while I am a sucker for launch coasters and there is no denying that Mummy is the more forceful ride, it's true coaster bits are incredibly brief and filled with really weak flashing graphics or whatever you want to call them. Maybe I'm missing some reference to one of the movies, but those flashing graphics seem painfully cheap for an otherwise brilliantly themed ride. Arguably the best parts of the ride are not even the coaster portions, such as the fire ceiling.
Everest, on the other hand, depends much less on special effects and more so on the physical themed structure and the coaster itself. Ultimately, the sheer size, speed, and duration of the ride, especially the final coaster section that begins with the large drop down the mountain, gives it the edge for me.
Revenge of the Mummy, by miles. Love it! You can't have too much fire on a roller coaster.
Actually that's not really true, is it? The Scenic Railway at Margate probably had a bit too much... but ordinarily I mean, coaster + pyrotechnics = good!
I haven't been in 15 years, does all that stuff still work?
Anyways, I think it made my Top 5 at the time. Granted I was still quite basic back then, less than 100 creds, but even now it's still firmly in my coveted Top 10%.
Expedition Everest on the other hand didn't make the Top 10% even in 2007 and you know me, if it doesn't make the Top 10% then it doesn't get ranked at all, it's just one of the 'other 90%' of coasters that I've ridden. Technically then, according to my weird personal ranking system, Expedition Everest falls into the same 'tier' of coaster as a wacky worm.
Or a Volare.