Whenever this topic comes up, I immediately want to say TP because of its reputation, but I must be fair and realize that SS gives it a run for its money. Both SS and TP have easy enemies, easy puzzles, fortune tellers to help find secrets and a partner that reveals the answers to many problems. But honestly, I'm gonna have to go with SS. TP's overworld gave some challenge in regards to finding secrets whereas SS's dowsing and almost completely linear path eliminate any chance of failure. While both Midna and Fi are pretty abundant in giving clues, Midna doesn't always have something to say about the situation while Fi knows just about everything there is to know in existence and will continue to let you know that she knows what you already know. And if that's not enough to sway a Person, we have SS's gossip stones which essentially tell you how to beat the game.
I've seen people struggle with TP because, as hard as it is to believe, some people just aren't good at Zelda games no matter how easy Nintendo makes it for them. (Hence why they keep thinking we're idiots) But when it comes to SS, even if you suck at figuring Zelda games out, there's a strategy guide within the game... about three of them actually. The only arguable advantage it has is combat which you might say is only more difficult because motion controls are more difficult to master than button pressing, (I Personally found little challenge in SS's combat) but other than that SS is definitely the easiest Zelda game, imo.