It depends on your age...for plain colors I like Geoffrey Beene.
I'm a bit on the young side for my company (average age is like 45)...so I wear alot of colorful striped shirts...they're more going out shirts...but they look nice tucked in with dress pants. For those I go with Claiborne, CK, Polo, BR, Lacoste, or Express. I've kinda cut back on shopping at Express. They have nice colors and designs, but the quality sucks....though I find their premium shirts last alot longer than their 1mx shirts. The 1mx shirts looked like crap after about 4 washes.