Alexander hardly has the depth and breadth of time to match up to the sheer number of accomplishments. He built Superboy Prime's armor and created a device that allowed him to recreate the multiverse and merge/alter the individual earths as he saw fit, which is pretty much the most ridiculous thing in comics (though needed a lot of different elements to power it, heroes from a bunch of dead universes).
But Reed beats him on sheer consistency and number of feats. You're talking about a guy whose workshop is full of weapons capable of killing Celestials, who has Galactus-stalemating armor, who traverses the multiverse with ease, who designed a device to strip Galactus of his power, etc.
Da guy is crazy, and consistently THE craziest guy in comics. Like the whiz, nobody beats him save for perhaps a few one hit wonders like Alexander Luthor.
I don't think there is any 'arguable' about it. He is much, MUCH smarter than Thanos, and he's proven his superiority over Doom time and time again.
And yeah, because Alexander Luthor has one or two things only under his belt, I guess he'd win. Because his one thing is huge. But I'd have trouble giving him the win over Reed, who really just pulls whatever he needs out of his @$$ whenever he needs it.