For general reference, see:
StickMiners - overview of low-power 'usb stick' type mining hardwareWhat is the best USB Asic?
Depends on your needs/desires.
I know it wont be amazingly profitable, but I wanna play around, maybe make a few $ a month.
Probably not going to happen - maybe a few $0.01/month and that's gross, not net - subtract electricity to realize you're not profiting at all
Is there any good Scrypt based USB Asic?
There's only two in a StickMiner form - the DualMiner USB and lketc's offering.
What is the best currently most profitable USB Asic?
Looking at the HEX*Fury but its pretty expensive £96.
The hex•fury, along with the NanoFury 6, are the fastest - but probably not the 'least unprofitable' due to their high prices. The Avalon Nano and NanoFury 2 models can be had for reasonably cheap (new / off of e-bay), or go for a NanoFury 1. They also require far less cooling than the 6-chip boards.
Push comes to shove if you just want to play around, find a good deal on Block Erupter USBs. They're very slow (by today's standards) but easy to use for just learning. Because you can buy a dozen on the cheap, you can also start messing around with overclocking, modding, etc.
If you're not in a hurry, I'd say wait for the results of GekkoScience's attempt at making a StickMiner based on a Bitmain chip:
GekkoScience BM1384 Project Development Discussion Also a good USB Hub for multiple miners?
Look for a powered USB hub using an MTT chipset (multiple transaction translator).
If you need to run a lot of devices or ones that require higher current draw, search these forums for some specific models that are easily modded to be powered using a computer PSU - but at that point you have to seriously consider whether you want to get into that
