With only have three slots for Elites units and with Fire Dragons being a prime choice for at each one of there slots.
That leaves either one (you might want two units of Fire Dragons) or two slots leftover.
Which due to the lack of Close Combat abilites of Guardians, means either Striking Scorpions, Howling Banshees or Harlequins.
Oh I like Storm Guardians since they can get you some more Fusion Guns but we all know that their Close Combat Ability is only good against Imperial Guardsmen, Tau, or other Eldar Guardians.
How Striking Scorpions do have 3+ Armor but they cannot Fleet and while they have the better ability to wound Space Marines they do not take away the Marines armor save.
Howling Banshees can due to their Power Weapons but they need 5+ to wound so their are some issues there. But they can get Counter-Attack and with their high leadership will get that extra attack giving them usually 3 attacks each so will cause some pain.
Though their biggest issue is staying alive long enough so that they can get into Close Combat.
Actually you would want them to be on the receiving end of a charge. This way they get to do their Overwatch fire and then roll for their Counter-Attack. But this is not easy to get since smart players will just shoot them.
Which bring us to Harlequins, they cost the most but they are very deadly. But they do only have a 5+ save and you really need to make us of cover or upgrade to a Shadowseer which makes them even more costly.
My first army was a Harlequin army and there was a reason why GW nearly killed them off. When use right they were nearly unbeatable. I played many games with them (when I could talk a sucker into one) and I rarely lost.
They have changed since the mid-1990's but they can bring some major Close Combat capabilities to your army. But in order to do so you will not be bringing something else due to their high cost.
Plus getting them into Combat is also an issue. Since even with a Shadowseer if they are seen a Invul 5+ save is okay but they are still Eldar and some of them will die.
Which brings us to the main question.
For 6th Edition which unit is a better choice as your Close Combat unit?
For me I am going with Howling Banshees, the reasons are they do have Power Swords, the Exarch can buy the Triskele which gives her some nice shooting abilities with a AP 2 weapon. Plus they can Counter-Attack.