TSCApparel.com offers a wide variety of blank t-shirts and they also offer relabeling services.
Which is the "best" would really depend on your market and what type of blanks you feel your customers would prefer.
For some people, 6.1 oz t-shirts would be too heavy, for some, it would be just right.
If you're looking for a specific brand, and you aren't sure which would be the best for your needs, I would suggest that you try out a few different blank t-shirt manufacturers to see which feels "right" to you.
Some you could try would be:
American Apparel (style 2001)
Gildan Ultracotton (style 2000 - this is usually the t-shirt screen printers use in their special offers they post on their site. But you should always ask to be sure)
Hanes Beefy Tee (style 5180)
BareApparel (used by high end MMA brands)
ProClub