Culture, without a doubt. It takes longer than any other Victory than Time (obviously), will put you behind in amount of cities, strength of army, and technology. Thus typically puts you in a precarious position where you're hoping the nearby warmongers decide not to gobble you up and instead go for someone else. You also must plan for your Culture win from the first turn, any straying from your goal will put you behind, possibly enough for your win to slip out of your grasp.
I don't think Time holds a candle at all. The only thing more difficult is simply dealing with pressing the Next Turn button until it's over. You have way more leniency when pursuing a Time victory, which is essentially a Domination victory that you hold off taking that last capitol. You can pretty much do whatever you want, however you want, and after you get to the point where the enemy Civs are crippled/outright destroyed you can just sit back and hit End Turn to your hearts content.
Diplomatic is the easiest, since it just requires someone to build the UN and for you to have enough coin to buy out all the city states. Even if you're not going for the victory condition at all you can coast into it by accident when an AI builds the UN, so long as you can scrounge up the funds.
Next easiest would be Domination victory (followed by Time) as crushing the AI isn't particularly difficult so long as you don't let them run away on a high difficulty. All you have to do is take their capitols, not to mention each conquered civilization will give you a large injection of science, gold, etc to make the next war that much easier. After a certain point its a forgone conclusion you're going to win. You can also achieve a Domination victory quite early.
After that I'd rank Science, as it requires you to actually be ahead or on par with the top AIs via technology in order to reliably win. If you stay small and tall then you run some of the risks that Culture wins run into concerning military strength vs wide warmonger civs. But at least here you should have a tech advantage with your units, or at least equal standing. Science win can come much earlier than a Culture win, but not as early as a Domination win. Going Science-focused can also open up the possibility of going for a quick Domination, particularly if you're considerably ahead of most or all of the AIs.
So I'd rank them as this; Diplomatic -> Domination -> Time -> Science -> Culture.
