I have pretty much every Harry Potter Lego set ever, and have them all set up in my room, joined together and modified by me, making a gigantic, one-of-a-kind Lego Hogwarts and its grounds. It is beautiful.
My laptop and my phone. I feel like almost everything else I have is replaceable, even if it would cost a lot of money. But I have work on the computer that is irreplaceable and I use my phone constantly for email and other stuff, and if I lost either I would have major problems.
More superficially, my comics (technically it's the trades and hardcovers; I don't really have that much in the way of comic issues). I have quite a few books and some being out of print would be hard to replace. TOG probably has more than I do but I really do have a lot.
Why am I not currently looking at a series of pictures of this?
I find this amusing. He has three children, and says his laptop and phone are irreplaceable.