well a good amp in what sense... depends what your trying to do...
if you just trying to amp your factory speakers...an amp will probably not do much for you... if you plan to swap out your speakers whats the best amp is still not really a good question.. Its all about the combination of the amp and the speakers that makes the difference...
you want to get something with enough juice to run your speakers...for example.. lets say you get 4 new kicker speakers which operate at (100 wats peak <--- ignore this rating and (this is the important one)---> 50 watts RMS well then you need an amp that going to provide 50 watts rms to those speakers in order to get the best sound out of them...
So if your going to amp 4 speakers that are 50 rms each you need a 4 channel amp with at least 200 rms... (200/4= 50 each speaker)
You also want to match the Ohm rating on the speakers and your amp..for example an amp that runs at 600 rms @ 2 OHMs runs at 300 rms @ 4 OHMs..so if you get (4) 50 watt rms speakers that are 4ohms then just realize that if the amp is rated 200rms @ 2 ohms then pretty much you will be providing 25 watts (half the power) to each speaker.. so it will not sound as good...
Just do your research..even if you plan on doing it yourself..go to like best buy ask what they recommend and pretend your going to do the install there so they can tell you all the information you need... make sure you speak to someone from geek squad and not a sales rep as they know nothing about music equipment most times...
As far as brands..I would stay away from non namebrand music equipment... trust me in this case. less expensive is always a disaster...
with amps
stick with KICKER, JL Audio, MTX etc...with speakers... ALPINE, KICKER, POLK AUDIO, etc... every brand has almost always 3 series of speakers..there regular series which usually starts at from 69-89 a pair... there mid series which usually goes from 100-130 a pair and there top series which goes from 130 up a pair... just know in audio equipment you always get what you pay for...