You have a few choices, depending on your price range:
The best khakis, far and away in my experience, are Bills Khakis. They wear like iron and are extremely well made. They come in two styles, the M1 (baggy) and the M2 (less baggy). Both styles come either pleated or flat front (I strongly prefer flat front for khakis). They do both a twill (the original) and a poplin (along with several other styles including canvas, corduroy, and a few others). Several colors are available (including some seasonal and limited edition colors - I think they are doing a cotton poplin in their version of Nantucket Red this season (called "Weathered Red"), but it may not be available everywhere). You can purchase them off the Bills website here: http://www.billskhakis.com/index.aspx
. Even if you don't buy from the website, it will give you a great idea of the range of their offerings.
I've also purchased them from Hansen's Clothing (link here: https://hansensclothing.com/default.asp
). The great thing about Hansen's is that they will hem the khakis for you free of charge. You can find them on several other websites, as well as in many local menswear stores (see the Bill's website for locations).
One word of caution. If you have them hemmed prior to washing, be sure to have them hemmed on the long side. In my experience, they will shrink a little bit in the legs (not the waist, however). Hansen's automatically hems them a 1/2" longer to account for this.
At regular price, they are on the expensive side (around $100). You can also find them at a discount at Sierra Trading Post (limited selection and sizes, however). I don't mean to sound like a shill for Bill's, and I don't have any relationship to the company. I just really like 'em.
Another choice is Charleston khakis. They get good reviews from the trad folks, but aren't as widely available as the Bills. Ben Silver sells them, at least in a "Nantucket Red" variety (BS calls these "New England Red"), and I recently purchased a pair. They seem pretty nice, but I haven't had the opportunity to wear them yet. Ben Silver may have other colors, as well, but I haven't really looked.
If you're more budget conscious, I'd suggest LL Bean. Their Double L khaki is a very good buy for the money, and come in a wide variety of colors. You can also look at J Crew (also a wide variety of colors), Polo, and Brooks Brothers (in fact, most menswear companies have their own khaki). I just think LL Bean gives you a terrific bang for the buck.
Undoubtedly, there are designerwear khakis out there which come at a hefty price, but I don't have any experience with these.