First off, this really needs to be on the General 40K forum. Rules and Questions is for questions about the rules.
This probably isn't the answer you want, but it's the best one there is in my opinion: The army doesn't matter, it's the person who playes it. 40K really is a pretty well balanced game system and every army has a reasonably good chance of success.
Watch games, read battle reports, talk to folks who play, and try to get an idea of what each army has to offer. Take some time to sit in the store and read through the codeces (they won't mind if they know you are picking out one to buy).
For what it's worth Eldar are a collection of specialists. Each unit has a fuction in the army, and they are one of the best in the game at it. However, if you try to have a unit work out side of it's specialty you're asking for trouble (and a loss). This is a big part of why Eldar are seen as a "Veteran's army", because the player needs to have a good idea of how the game works to understand the toll of each unit. I don't think this is neccessarily true, but it does require a new Eldar player to do some extra preperation before they start playing.
Argueably the easiest army to play is Vanilla Space Marines. No special chapter rules, must Space Marines straight out of the standard codex army list. This is where the majority of new players start, but there's reason anyone has to follow the herd.