Which one is the best and why?

Versatile enough for Jazz, Blues, Reggae and Funk.
Budget $600 usd
New or used, it doesn't matter.
Gear Warwick Corvette Standard (Germany)
Buying it online.
I'm primarilly on the other end of the spectrum... large, heavy, all tube amps from the 70's... but i can totally appriciate the usability of a micro-amp.

Of the ones i've used, i've enjoyed:

-Genz Benz Shuttle 9
-Ampeg Micro VR (lower wattage, but great tone)
-GK Fusion 500
-Markbass LMII
"Punk Rock should mean freedom, liking and accepting anything that you like, as sloppy as you want, as long as it's good and has passion."
The LMll is a wonderful amp.
Try, also, Carvin's BX500. It's a lightweight tube hybrid, using a 12AX7 tube in the preamp. Rave reviews abound.
...it was bright as the sun, but with ten times the heat
The GK MB series is quite good, but my vote goes to Markbass. You can pick up a used Little Mark II or III for $400-$450 and they're absolutely killer. Ultra transparent, super light, and as loud as you'll ever need it to be. Genz Benz would also be a great choice if you could find a used Shuttle within your budget (or stretch it by $70 or so and get a new Shuttle 6.0).
Composite Aficionado

Spector and Markbass
I have a MB200 head, great head Consecutive E had one too, but it doesn't have an effects loop which is why he traded his for the Ampeg VR. Markbass is nice, but I'd say if you got a MB200 for $250, a Sansamp and a 2x10 you'd have a great micro rig probably for around $500

Are you going to make an entire mini rig? Because the head is nice, but If your going to be taking multiple big cabs and such I'd not bother narrowing down your head choices to the small ones.

I have mine for practice, and a micro rig that runs at 8 ohms so 100 watts, and full 200 through a 1x15, and 2 1x15's that make having it kind of pointless, but by that time, I switch (when its repaired) to my Mark IV and run 2-3 15's
+1 for the Little Mark II
1978 Peavey T-40 -> Ampeg Micro-VR - > Ampeg SVT210AV + Ampeg SVT-15E
SWR Headlite.
400 watts and it's all self-contained. no need for a cab or anything.

the single volume knob can cause problems though.
"I am hitting my head against the walls, but the walls are giving way." - Gustav Mahler.

Quote by AeolianWolf
absolutely what will said

Yay, my first compliment!