What I've heard of Tom Araya's bass tone I'd say it's kind of bland. Same goes for Trujillo. Dave Ellefson at least has mids enough to be heard.
After I posted I got curious enough to go digging through Youtube for some isolated bass tracks (mostly ripped from Rock Band games).
Things I noticed:
Ellefson: I'm not familiar with the Megadeth catalog, but I'm not surprised Ellefson gets a lot of respect from metal bassists. Impressive. Some dumbass in the comments said "This isn't really bass playing, it's rhythm guitar on the bass". Whatever, it's awesome
Araya: By far the weakest Big 4 bassist. I don't really like his distorted tone, and his playing works fine with the band but that's the most I can say for it. Lombardo really carries the rhythm section in this band. I think Araya's vocals are his more important contribution.
Bello: I always enjoy Bello. The "Caught in a Mosh" isolated track was surprisingly sloppier than I expected from the full mix, where it sounds great. But then, so is Entwistle. Later recordings seem cleaner, not surprising his technique has improved in the past three decades!