Let's address the elephant in the room first: Android. If you don't have an iPhone or don't want to switch to one, stop reading right now. Cause you need an iPhone for pairing an Apple Watch. Yes, there's a cellular version of the Apple Watch (and Family Sharing), but you still need an iPhone to set it up.
So Series 6. It looks like Series 5... and Series 4... and the new SE model. It comes in the same two sizes: 40mm and 44mm. Small and big. Take your pick. Exclusive to Series 6 is a blue and Product(RED) version. I'm more of a stainless steel guy, but the blue aluminum looks really nice. A good dark blue that sometimes looks black in low light.