Best way for connecting this redstone vertically?

Solution 1:

If low latency is more important than space, you can go straight up in a mostly 1x2 space, with a small section of 2x3:

If space is more important than low latency, you can replace the repeater with a vertical double NOT gate, which would lower the maximum space from 2x3 to 1x3:

Solution 2:

I finally got it like this: Every second floor is a repeater with two redstonetorches (idea from [Russ Whitchurch] here)

Solution 3:

If it is just down I use a dispenser into a block(connected to a Piston) then a piston fires releasing it and it falls onto a pressure plate on top of a fence (at the bottom)!