Vayne over ashe in the easy department. She has a get off me move, speed boost and roll to evade, invis, great auto attack, less concern with managing mana, can give chase...yeah vayne is nub friendly and just beastly period.
I would say Ryze for AP. Tanky for a mage, has snare, and good burst dmg.
Warwick is probably an easy jungler to start out with.
Taric for support. His long cooldowns means new players don't have to worry so much about spamming spells.
GP maybe is noob friendly for bruisers. Quite tanky, has heal, escape cc, easy to last hit.
Not sure abou the easiest tank though. Maybe Malphite? Just ult team from a distant to initiate.
It may just be personal taste, but I have opposite opinions for some of this new player stuff:
When I started with Ashe due to the tutorial using her or whatever, I thought she was super hard to use. I probably lost a couple of bot games with her, and she's so very squishy. Once you start learning about strategic slows and using her crit passive maybe she's easy, but she didn't strike me that way.
Warwick may be easy for killing jungle monsters, but he is so slow to move around I am not sure I would recommend him even for new people...maybe so?
Vayne I do NOT recommend for new players. She was one of the first expensive champions I bought (maybe after Shaco and Caitlyn), and it seems like there are lots of terrible Vayne players, or players who are sometimes good but quickly become terrible if they don't get a good start in the game. The thing is that Vayne is so weak, people who are unfamiliar with her will burn her mana for rolls, and/or not pin people to walls, and end up dying in like 3 hits. It can be even worse if they try to start with doran's blade and their lane partner doesn't have a heal. Plus her range is not ultra long, so attacking minions might put them in the lane of some tanky gap closer. GAF games, non-GAF games, whatever, seems like beginner Vayne players will have a tough time.
I don't feel that gangplank is tanky until you get items on him. I have seen so many people saying he's easy to use, and tried him so many times myself, with different varieties of runes, and it never works. Now maybe I'm one of the worst players ever (possible), and maybe going armor + CDR + heavy arpen is a bad idea on gangplank (possible), but I always seem to be running out of mana and/or being squishy and/or not farming well early game. I haven't tried him solo top much, however.
I did start playing by trying to learn tank with malphite, so I'll agree with malphite, annie, and maybe ryze. You need to learn the quick chain constant spell seequence for cooldowns with ryze, however, something I still haven't figured out efficiently. And of course if you play tank, as I did with malphite, your whole team may scream at you how the loss was your fault, and you weren't tanking enough, and you didn't initiate properly, and you stole a creep or a kill, etc. etc. Tank and support are rough roles if your teammates are screamy, and it might put some people off the game.
I'm trying to think if there is anyone else good for new players. Maybe garen with his lack of mana, though again for some reason I couldn't figure out how to play him. Perhaps caitlyn/kogmaw/tristana depending on who is free that week, for the extra range over vayne, or graves for the built-in armor and magic resist.