I'm not going to be reiterate "pick your favorite" because that's often redundant.
Between the armies you mentioned, I get the impression that you want to play shooty armies. Since eldar necron tau and dark eldar are all shooting armies. If you don't like to focus on shooting, I would not recommend you pick them.
Tau is the stand and shoot army. They have incredible volume and strength of fire, and plenty of support to help that shooting get even better. The jet pack suits and fast skimmers also provide some mobility, although tau is far from a mobile army, as your main army will be relatively static firing platforms. They are not too fragile, but will fall apart to even guardsmen in close combat.
Necron is getting a new book soon, but necron is also a very shooting based army. However, they are considerably more durable, with reanimate on protocol and across the board higher toughness and saves. They also have very strong vehicles, as well as impressive volume of fire. On top of that, they also have fantastic flyers which was the bane of 6th edition, as well as the ability to drop troops all over the board with said flyer. Necron is also considerably better in combat, with actual dedicated combat units! Like tau, necron will struggle when it comes to transporting large amount of troop across the board, but with a new book coming out, I would wait for it before starting necron.
Dark eldar is one of the most mobile army in the game, with what is possibly the best transports in the game. Venoms and raiders are fast, and packs a punch with their shooting weapons. Massed poison shooting will put a hurt on high toughness units, and dark lances are easy vehicle killers. Their weakness? Fragile... Almost all their troops are guardsmen with better shooting. Transports are open topped, even the heavy vehicles are open topped with mediocre armor values. 2 hull points on Venoms mean while they are strong, even bolters will be shredding them if you are not careful. Not mentioning if someone drops a flames template your guys inside takes d6 wounds ignoring saves...If you do play dark eldar, you have to play like they are in the fluff. Be fast, lay down volley after volley of hurt, and go into cover. Play them well and they will be one of the most feared army in the game.
Eldar is the most hated army on the Internet in 6th edition, because almost solely on the wave serpent, which can put down an obscene amount of firepower. Eldar has a lot of options, for fast transports, to walkers, to monstrous creatures, to terminator equivalents, to flyers, to heavy tanks, to much more. In 7th, the balance swung back more or less, and if you want a codex where almost everything is viable, this is it. Whie tau have things like vespids, necron and their praitorians, eldar can almost play anything in the codex and get results.
If you are still torn, ally. Dark and regular eldar are battle brothers, and necron and tau can be convenience allies. If you are really torn, you can try allying.