You really can't go wrong with any of them.
My favorite is the DC-10 though.
I've had 2 DC-2's and my only real complaint at the time was that the Contour only works on channel 2.
The best way to decide is to ask yourself how much clean headroom you need, and what type of gigs you want to play.
I can play my DC-10 at bedroom volume and usually I get a really good tone that only gets better as the volume goes up.
If you don't play clean very much, or if you like some hair in your clean tone, then any of the DC's will be perfect.
I like pristine, Fender like cleans at LOUD volumes, so the DC-10 is the one for me.
The DC-5 holds up pretty well, but my drummer is really loud (but incredible) and I always found myself wanting more headroom.
The DC-3 is really good too for a band if you use a Thiele cab under it for more thump.
If I were looking for one, it would be this one: http://cgi.ebay.com/Mesa-Boogie-DC-10-Combo_W0QQitemZ290263391919QQcmdZViewItem?hash=item290263391919&_trkparms=39%3A1%7C66%3A2%7C65%3A10%7C240%3A1318&_trksid=p3286.c0.m14
Here are a bunch more: http://instruments.shop.ebay.com/items/_W0QQ_dmptZGuitarQ5fAmpQ5fAccessoriesQQ_fromZR46QQ_sopZ10?_nkw=mesa+dc+-case&_sacat=38072&_fromfsb=&_trksid=m270.l1313&_odkw=mesa+cabinet+-case&_osacat=38072
If you go for a DC-5 combo, let me know. I might part with mine. It has a few extras that make it really sweet.
Bias pot
Better tone capacitors
New power supply capacitors
New LDR's for channel switching
Cathode/Fixed bias switch
Better reverb
Just re-covered in burgundy tolex