I have a Timney 3lb drop in trigger assembly in one of my AR's, which as mentioned above is a single stage trigger. I have Rock River Arms two stage match trigger group in my other AR.
The Timney offers a fairly light, crisp, consistent 'break'. I like the way the trigger assembly replaces all of the stock trigger parts, except for the two pins. The way the trigger installs prevents the stock pins from rotating in the receiver holes, which is known to cause wear on the pin holes of a gun that is shot a lot.
For the money I think the Timney is a tough deal to beat.
The RRA two stage match trigger also offers a nice light 'take up', then a crisp, clean break. For the money, this trigger kit offers a vast improvement over the stock trigger. If you are on budget and want an inexpensive trigger upgrade, I'd highly recommend considering this.
I have not shot, or pulled the trigger on a Geissele trigger, but I have heard all sorts of positive comments on the triggers they offer. The Geissele triggers are high end, premium triggers, basically you get what you pay for! If you've got a fair bit of $$ in your budget, I don't think anyone would be disappointed in any of the Geissele triggers.
Let's face it, the stock trigger 'suck's for long range accurate shooting, or improve rapid fire accuracy.