Well, that's a complicated one. And it's 3 questions to be precise.
First of all I am a bit reluctant to name the best WM armies as someone could be tempted to just buy those to have a winning army. But I'll still try to answer.
Armies for newbies:Chaos (as it is cheap to buy and it allows some mistakes)
High Elves (same reasons)
Tougher Armies to play against: Skaven (as there still are some troubles according some rules (Screaming Bell, Gutter Runners) and not balanced pointswise
Daemons (as the Daemon army I mostly met is Claus's one

and the army has no weak points imo - in skilled hands)
Lizardmen (special commanding rules and good troops for a good point prize - mostly)
High Elves (high quality troops and cavalry with Command 10 as super bonus)
Difficult armies to play: Undead (no initiative and restricted number of characters - but a very good army in skilled hands)
Dwarfs (no cavalry - but Command 10 bonus - and can still be a tournament winner if played by a good player with a fitting tactic)
Kislev (Restricted possibilities troopswise - Warwagon very much pins the tactics down - but I had a player at a EuroGT in my team who won every single game with them - in a defending scenario)
So as said before at Warmaster it's not the instrument - it's the player (and the dice a bit)!
Greetings,
Gerald