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Which base of operations would be the hardest to infiltrate and which would be the easiest to take over?

FF Baxter Building
Castle Doom
Xavier School
JLA Watchtower
Avengers Mansion

I'm unfamiliar with the JLA Watchtower and the Avenger's headquarters are in multiple locations (mansion?) so I cannot answer that either. Dr. Doom's castle is quite penetrable and despite the cloak that once surrounded Xavier's school, it has far too many holes to be even considered a contender in current times.

I remember only one occasion when FF's base was infiltrated (I'm not a true FF reader so I have little to go by) and that was only because Invisible Woman allowed him to (Wolverine in Infinity War).

Well the JLA watchtower is on the moon, so it is preetty hard to get there If you are not Lex Luthor or god or something, but what about the Batcave, isn't that hard to infiltrate (at least after the Bruce Wayne murder thing?)