What whatever the heck you want.
None of us know what you need in your pokedex, what you need, what your overall priorities are, etc. That's like asking someone what you should eat for dinner. There is no right answer.
If you're looking for a practical buddy that has a future purpose, walking a Larvitar/Pupitar/Tyranitar so you can save up candy for the upcoming community day. That's what I'm doing. But my pokedex is complete and I don't need anything else.
My advice would be to never walk scyther. It's a waste of km. But again, if getting a scizor is important to you then go for it. Once again illustrating the futility in asking strangers what you should be walking.