What's the easiest/fastest way to kill a tallboy?

Razor Blades and Sticky Bombs both definitely work. Alternately, you can just blink up to them and stab them for a one-hit kill if you're unnoticed on the approach, which is extremely satisfying.

Around the time you run into them you can have Agility 1 and the Stealth boots which basically allows you to sprint up to them, jump to their height and perform an assassination without being heard. There's no need to jump on any objects first if you are on the same ground as them: Agility 1 allows you to jump right up to them.

Doing this you can Blink to behind their feet instead of mid-air so there's no drop assassination aiming involved and it's possible to kill tallboys in rapid succession with little to no chance of missing. The stealth boots are nice for catching up to them while they're walking since it allows you to sprint silently. You'll have to find the blueprint for them in the game and they are by far the best upgrade there is if you enjoy kniving.

Beside that, you can possess one and steer it to a place nobody will see you as you come out and jump-assassinate them straight away. When assassinating one of the whale oil tanks seem to produce noise when they shatter against the ground, but this noise isn't heard by enemies.

If there's any spare tanks of whale oil around - and there often is one or two, you can always pick it up and throw it at the tallboy. Make sure to at the very least, catch the tallboy's whale oil in the (rather large) blast radius. Not only are you depriving the enemy of a resource, but you're avoiding using your own!