I don't even like them anymore, and even I can tell you that Slayer has retained more of their core loyal audience than any of the other big 4. Slayer fans are just a whole new level of "they can do no wrong" fanatical.
And I have no idea why. The quality of their work took a steep nosedive after Divine Intervention (which is already the weakest of their first six), and has only slightly recovered since then. Now that Jeff died and Dave was kicked out, there is no hope for a return to glory.
Of the other Big 4 bands, I have not listened to much of Anthrax's work. Metallica's debut was ok, the next four albums were great, I have not listened to the Loads, and everything since then is not all that good. Megadeth fared a bit better, as they released a few good albums after Youthanasia (namely UA and my personal favorite of theirs, Endgame), but the second half of their discography is still rather underwhelming. I'd say that, of these bands, Megadeth has the most potential right now, as two of their four most recent releases were pretty good (compared to what may be a similar number for Metallica, though their last four albums were released over a much larger time span, and 0/4 for Slayer, even though I am somewhat fond of World Painted Blood).