I'm using the default as far as I know, though it's pretty invisible most of the time.
For those of you having difficulty seeing the x-hairs you might like to try something. Drop the console and type:
bind mouse1 +attack;adjust_crosshair
I think that's the command anyway - if it does nothing, try google
If you like this you may need to add it to your config file to make it permanent. What it does is, each time you press your left mouse button, it changes the color of your x-hairs. There's 5 in the cycle (the same in the options screen if memory serves) but it makes it much easier to see/keep track of and if you're up against a terrain where the current colour is impossible just do a quick knife stab to change it.