my experience of playing against Harlequin with my Chaos (Slaanesh).
Solitaire is very, very, very good, a solitaire could easily kill my Daemon prince with riverblade and charging in with 11 attacks.
Venums will pop open like crazy, holofield or not. av10 opentopped just die so fast to anything. (my advice, don't bother with holofield, it will die, anything shooting at it will cause it to die)
Death jesters are kinda crappy, their initiative 4 is what killed them. Marines go the same time as them, and my Slaanesh stuff always go before them. (you might think they won't get charged, but they will get charged, any die so fast before they can blink)
The regular harlequins die to flamers like crazy, I wouldn't invest so much on it. (Yes, I use flamers alot, not just against harlequins, but anything)
The great harlequin is good, but no where near as good as the solitaire, but much better than the regular harlequins.
Harlequin jetbikes are actually pretty good, always spread them 4 inches apart. They actually get a 3+ armour save vs my flamers, which is why they are considered good. They have a 3+ armour save and a 4+ cover save, and they're fast, so invest in them more than the death jesters.