There is a lot of choices.
Tascam is now a new contender as they have finally tried to improve their drivers.. I would be giving them a second chance if this is actually true.
I have a Focusrite but I don't like the pre amps,,, the drivers are excellent.
Anyhow good luck.
Make a list of your requirements first.
.
What kind of connectivity do you require? and how many of each.
XLR, 1/4", or Combi jacks, RCA, line level, Instrument level ,MIDI, SPDIF, ADAT etc.
How many ins and outs do you think you'll need now and in the future.
Are they front panel or rear panel?
Balanced?
What do other say about the pre amps? Warning, this is subjective!
Are there peak level meters for each input?
Are there channel Insert jacks?
Are there separate controls for Monitor level and headphone level?
Is there a blend control for mixing Source with Computer ( DAW) ?
How many Headphone jacks?
Front/rear panel pad or line / Inst toggle swicthes or software driven?
Is it a metal box or cheap plastic? Is it light and portable or large and bulky, Rack mountable?
Does it have an on off switch?
Does it use Buss power or a power supply? Buss power can have issues with noise and Phantom power.
Does it have DSP effects built in?
Does it use a GUI mixer?
Can it be used as stand alone?
What are the Round Trip Latency (RTL) specs? Do you need low specs for real time processing?
Zero Latency monitoring is not the same as RTL. All interfaces have latency.
And most important of all , Does it have top notch drivers for your OS.
What is the word on support from the company?
Does it come with free software, An LE version of a DAW you would like to try?
Everyone will recommend the interface they have chosen, that doesn't mean it is the right one for you.
http://forum.dawbench.com/showthread.php?1548-Audio-Interface-Low-Latency-Performance-Data-Base/page13