Aside from the Atari 2600 - my favorite system is defintely the Tandy Color Computer ( or Colour here in Canada
) Its the machine I grew up with and I have all 3 major revisions - Coco 1, 2, and 3 including a 512k model coco 3. I have about 60 carts but no disk drive unfortunately but I use a homemade piece of hardware called drivewire from a website called cloud9tech. It allows you to hook up a coco to a modern pc with a serial cable and use your pc as a virtual disk drive to load coco disk images on real hardware. Some awesome arcade ports came out on the coco which many outside of the coco scene have probably never heard of.
I'm also a huge fan of the atari 8-bit systems and own 3 800xl's. I had one of these briefly in the 80's before getting the coco (since we had friends with a coco who would makes copies of games for free, as opposed to no one we knew who had an atari computer). Again, some fantastic arcade ports on this system and i love it.
Always thought about getting into the C64, used one in high school in the 80's (again, great arcade ports - that's what i'm all about)