They all look the same and they are not the same. Some you win most of the time. One of them you will lose for a fortnight and enjoy it. Here's the honest ranking.
FreeCell. Almost every deal is winnable, every card is face up from the start, and there's no luck in it whatsoever. If you want to win, and to know that winning was your doing, that's the game.
But "easiest" isn't quite the right question, because the games are pleasurable in different ways. So:
FreeCell — over 99% of deals are solvable. You'll still lose, because it's a real puzzle, but you'll lose to your own thinking rather than to the shuffle.
Yukon — no stock, everything visible, and you can lift any card with whatever's piled on top of it. Around 80–90% winnable, and it feels wonderfully unconstrained.
Klondike — the solitaire that came free with every computer. Turn one, unlimited redeals: something like 80% of deals are theoretically solvable with perfect knowledge, but you don't have perfect knowledge, so real players win around a third of the time. That gap is the whole game: you are guessing about cards you can't see, and sometimes the guess is unwinnable from the first deal.
Spider (one suit) — long, absorbing, half an hour a game. The one-suit version falls to careful play most of the time; it's the two- and four-suit versions that eat afternoons.
Klondike turn three — the same game as Klondike, except two out of every three stock cards are visible and untouchable. Winnable maybe a third as often. This is the version the card sharps consider the real one.
Golf — roughly one deal in ten. But each game takes two minutes, so losing costs you nothing, and the chains are genuinely delightful when they come off. Think putting practice, not the Open.
Forty Thieves — under 10%, and it doesn't get much better with skill. Two decks, build in suit rather than alternating colours, move one card at a time, and no redeal. Every card you turn from the stock is a card you'll never see again. Legend puts it in Napoleon's hands on St Helena, which is almost certainly nonsense and entirely fitting.
Every number above is a rough figure, and anyone quoting you a precise one is guessing. Win rates depend enormously on how well you play, whether you use undo, and how the particular variant handles redeals. What's not in doubt is the ranking: FreeCell is the most winnable, Forty Thieves the least, and the gap between them is enormous.
One thing worth knowing: not every deal can be won, in any of these games. That isn't a failure of your play. The table here tells you when a deal has become impossible, instead of letting you stare at a dead board wondering what you missed.
FreeCell, comfortably. Every card is face up from the beginning, there's no stock to draw from, and well over 99% of deals can be won. Nothing is hidden, so nothing can go wrong that isn't your own doing.
Forty Thieves. Two decks, you must build down in the same suit rather than alternating colours, you can only move one card at a time, and the stock never recycles. Under one deal in ten falls, even played well.
No — and this is worth knowing, because people blame themselves. Klondike deals in particular can be unwinnable from the moment they're dealt, no matter how well you play. FreeCell is the near-exception: almost every deal is solvable.