Looking at running on of these. This would be for a single player running multiple characters using 3.5 rules. Just finished running Return to the Temple of Elemental Evil for him.
So some questions...
1) Which one takes least prep?
2) Which one has the most straight forward plot (e.g. raiload dungeon delve etc)
3) Your recommendation? why?
1. Age of Worms is heavy on the Greyhawk nostalgia. Depending on where you set RTOEE, that may matter to you. The other two magazine APs start on the edge of the Flanaess map, and are therefore easier to transplant. The first adventure in all three magazine APs each have fairly large dungeons, while Runelords has several smaller dungeons (the final dungeon I would say is more medium size, but YMMV). So which is easier to prep depends on whether you need to spend your time on the setting, or lining up the stats to monsters and NPCs. Unlike the others, Runelords is the start of a trilogy, is set in Golarion, and is pretty tied into an important part of the history of Golarion, so if you do not play in that setting, transplanting it out of there could be a major prep challenge.
2. I would say that the overarching plot of the entire AP is most obvious from the beginning in Savage Tide, though it is not too obvious until adventure two or three, and just who is the ultimate villain takes even longer to become clear. Shackled City has a quest giver for the first few adventures, but the main villain and driving force/plot of the AP is the most obscure and difficult to discern, so the last few adventures may be one long instance of "who are these people, and why am I fighting them?" This weakness was noticed at the time and is still present even in the hardcover. AOW is about saving Oerth from the end of the world, but will likely appear initially to be about reassembling the rod of seven parts, if anything. Also, in AOW there is an adventure that almost certainly can not work as well as intended with only one player, though to say more would be to spoil it. As part of the Greyhawk cranked to 11, AOW also has a few call-backs to various adventures that appeared in Dungeon magazine around the same time, which might go over your player's head completely.
3. Without more information, it's hard to answer this question. With 1 player / multiple PCs, I'd guess SC or ST are the best options of the four you offered. The ideal choice may be one of the other 3.5 Pathfinder APs. Curse of the Crimson Throne, Second Darkness, and Legacy of Fire may be worth considering, as each is both self-contained and much less tied to the Golarion setting than ROTR. SD needs a bit of work particularly in adventure 5, and some expectation management in the early adventures, but this is actually easier with only 1 player. ("Hey, you with the heroes: This AP is about saving the world, not the small time scam you started with in the starting town" might be enough.) All six volumes of SD are still available for $5 each here. SD may be easiest if the player is willing to try an all-elf party.