I've been researching on-line, but wondering if anyone has done the ground work and tried a few. I'm thinking 5-6 watts.

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I like small low powered amps. Unfortunately, the low power amp I have is 22 watts  so that's probably more then you're looking for.  Mesa Boogie Studio 22.  It can get fairly loud and works fine on stage as long as you can get it close to you or the band is professional and knows how to keep the volume under control. In the studio, set the master low.... 3 or 4 and let the pre's do all the work. Sounds good.
 
A fellow I went to high school with, back in South Jersey, has a company which makes small amps. http://www.guytronix.com/  I think they are mostly kits ... so if you're into that aspect, you could have a truly custom rig.

Guitarhacker
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No doubt, for me it's my 70's Fender Champ. It's a great sounding little amp that breaks up nicely when cranked. I put a Weber speaker in it awhile back. I've never had issues with it, not bad for a 40 year old amp.
There's a lot of choices and variables, I guess it depends on what your going for. I recently saw some cool videos on the Orange Tiny Terror.
 
Rocky

So subjective, I love my JCA22H (20 watter), my Orange Micro Terror (also 20 watter) and my Gretsch 5222 (5 watter but I did mod that one on the pcb to "open" it up as well as replace the non-acoustically invisible terrycloth-like speaker grill cloth)
Best? Depends on what you're looking for...

Hey, go for the gold ... I have a Fargen Mini-Plex that is only 7 watts or so, and it is killer.  It emulates 3 different Marshall circuits with a switch, and you can swap among several different power tubes to get the sound you're after.  Tain't cheap, though.
 
I had a silver-faced Fender Champ that I never got a good sound out of, and tried an Orange Mini-Terror that sounded fizzy no matter which tubes I used in it.  Also had a Bad Cat Mini-Cat head (5 watts) that I could never get to sound right.  YMMV.

the best?
 
how much money are you going to spend?
 
low watt amps are all over the map,
trying to guess based on no information is hard.
 
 
at least give us a price range.

$269
http://www.zzounds.com/item--BLCHT1
 
$750
http://www.baroncustomamps.com/snott.php
 
$849-1049
http://www.emerysound.com/Superbaby.html
 
 
 

Most importantly is the type of music you play and which type of distortion you prefer. Also what type of guitar and Pick ups. 
I'm Country Rock, 60's Rock and Blues.  
I have a  22 Watt 72 Fender Princeton which is my main amp for over 25 years now, live and studio. It has been modified and upgraded.   I also have a 7 Watt 64 Gibson Skylark which is not really great for live but I use it for studio stuff.  7 watts is a little whimpy even when miked. I beleieve 20 watts is the perfect amount for having an gritty edge but still clear punch. I want to hit the strings and feel that percussiveness with string mutes. Small amps under 15 watts have no punch. Might as well use a distortion pedal.  
I've owned most major brands of amps over the years and once I "knew" what sound I wanted, I got the right amp. 20 - 30 watts, good tubes and the right speaker. I use Celestion Vintage 30's. 
 
I owned a music store back in the 90's and if you like the Marshall sound, look for a 10 watt Samick guitar amp. They were I dentical in tone and a lot of the parts as the Marshall 10 watt Valvestates. I think Marshall farmed out the Valvestate amps to the same plant that made Samicks ( Korea) 
 

 

 

I was checking out the Marshall DSL 5 this morning. That looks like a contender.
 
Rocky

This is my current favourite low power amp. Very much like a VOX AC-4. Right now I'm working on its big brother.
 
https://www.dropbox.com/s/quzpyxmgaw0k1pv/_MG_0096.JPG?dl=0