I am the same way, and my grail would be my MBX as well. I have all of the games and overlays except the Trolley game (can't recall the whole title). We had a 99/4A when I was growing up, and I LUSTED after the MBX. Voice control! Are you kidding! Alas, my dad could not be convinced to purchase one, even if one could have been found. Well, finally picked one up on eBay and it is just as cool as I thought it would be, although I don't have as much time to play as I would like. Also picked up loaded PEB at a pawn shop (who took that in?) and so now I can play Tunnels of Doom off of a floppy instead of the cassette deck I had to use back in the day.
I also am a sucker for add-ons and oddball controllers as well, and own many such failed products, like the ECS, the Vectrex light pen and 3-D imager, the 32X, the SMS 3D glasses and adapter, etc. I've always thought it interesting that so many of the games meant to highlight these products were baseball games. The MBX, ECS, and ColecoVision Super Action Controllers, at least, all had baseball games as their "flagship" titles. I doubt if this is a coincidence, and I suppose it reflects the overall popularity of baseball in the early 80s. Today I suppose all the games would be football games.