M&P
SIG
RUGER
GLOCK
I am no expert, by any means, but I would say the above list is solid. IMHO, I would say specifically:
Hipoints (for reasons already listed in this thread)
9mm and .40 cal would be my focus for many of the listed companies.
Some specific guns I have heard sell well:
Glock 17
Glock 19
Glock 26
Glock 22
Glock 23
Glock 27
Glock 21
S&W M&P9
S&W M&P40
S&W SD9
S&W SD40
S&W J-frames
Ruger LCP
Ruger LCR
Ruger SR9c
Ruger SR40c (just released - I am guessing this will be as popular as the SR9c)
Keltec PF9
Keltec P3AT
Colt 1911 (I'm guessing you could spend thousands putting various 1911s in your shop, but Colt's is one of the most popular)
These are just a few off the top of my head - again, I'm no expert