As Ct says, it depends on what you want and how you are going to use it. Here are some things to think about that might help you to decide.
Do you like programmable things - gizmos, gadgets, MP3 players and so on ? If so, and you like reading manuals, setting things up just the way you like them, then the Novatac might be for you - but if you prefer simple (take it out of the box and switch it on), one of the others might suit better.
Think about what method of control you prefer: everything on the EX10 is controlled by the tailswitch - whereas the Quark (especially the Tactical) involves a certain amount of head twisting.
Mode memory is another important factor: if you tend to switch the light on once and then use it for a period of time (for example going for a walk, etc.) then the simple interface on the regular Quark would be fine. However, if you are going to be switching on and off frequently (i.e. using it as a flashlight), then the EX10 would be better because it remembers the previous level instead of always starting on low.
If you have to vary the light level often (working on different tasks or in different environments), then the continuously variable output on the EX10 is the best choice. If you prefer to set your favourite levels once and then leave them, go for the Tactical Quark.
Both have the choice of clip or no clip, but on the Quark 123 you have to select this at time of ordering.
Finally, the Quarks give you a choice of tactical or regular interface, so that's worth thinking about too.
