I haven't had the greatest experiences with Monarch Computers. Their customer service sucks. They charged me for a cancelled order and it took me five weeks to get them to issue a refund. One of the chips they sold me stopped working after two months (was in another comp, not overclocked) and it was a major hassle to get them to live up to their 3 year warranty on the chip. I ended up having to send the chip in to them two times before finally receiving a replacement.
TCWO is not the greatest either. It looks like ground shipments have a very low priority with them and do not ship for a week after the order is placed. I placed a time sensitive order with them with two day shipping (needed it by the weekend) and the order was delayed for some unknown reason (product was in stock). Rather than "do the right thing" and upgrade me to overnight shipping for free, I was forced to foot the extra shipping cost.
Newegg is great! Definitely do business with them. If your order is delayed in shipping they will upgrade your shipping for free (like they should!). They ship out fast with no problems. I have not had to return anything to them, but based on my experiences otherwise, I'm sure it would go smoothly.
I'd definitely go with Newegg over both of the establishments above. And I'd go with TCWO long before I'd go with Monarch Computer (things are fine if the order is good, but if you have to return something or get a refund you are screwed).
I've never done business with Mwave.com, BestByte, KDcomputers or Vikingcomputers.