I am t-shirt fanatic!! Living down in the south that's pretty much all I wear (top-wise) and it's definitely the choice clothing of most people here, I probably buy on an average anywhere between 5 and 10 tees a month (not counting your basic underwear tees like Hanes and Fruit). The brand that fits me best is Buffalo, I like the fit, fabrics, colours and designs, I own probably dozens of their tshirts. Ben Sherman tees are probably my second favourite, especially the vintage series. Gap is probably my most used/bought brand for basics and layering - the fit is hit or miss but the pricing is very affordable. I also am quite partial to Guess tshirts (especially the pinup girls series - but their fit can be extremely uneven), I like Itsus for designs (but not prices), as well as Republic Clothing and Soul Rebel.
On the other side I absolutely detest generic department store brand tees - Polo/Ralph Lauren, Tommy Hilfiger, Nautica, Izod, most of DKNY & CK and such.