I'm 6foot3 and not very built so slim fit always fits me really well.
But when it comes to some basic clothing including white tees I've never been able to find something that fits me well. It's important to me that it fits well on it's own or under a cardigan or jumper. I've tried Fruit Of The Loom, Primark and Gildan and they were beaten in every single way by the Hanes slim fit t-shirt.
Not only did it fit very well, the quality of the fabric used feels more premium - and 3 months on has lasted longer than the brands mentioned above. Since it's slim fit it looks great under a cardigan too - something the other brands never managed since they fitted so loosely or became stretched.
The length from shoulder to hem was also very generous, especially given that I'm taller than the average. The neck wasn't too small in circumference so I felt I had room to breathe and the short-sleeve arms came half way to my elbows rather than showing off my shoulders!
They're a little more pricey than the alternatives (3x as much as the Primark branded ones) but definitely worth the money. I'll be sticking to Hanes until I find something better - or win the lottery!
I was pretty devastated when I found out that Hanes were discontinuing their T-Easy 4550; those t shirts had a lovely natural cotton finish and a snug fit.
Now we have three options: the slim, the tagless and the beefy.
My suggestion is that given they are so cheap is to try one of each and see which one you like - as a word of warning if you are generally a small - then try the large or medium slim fit t shirt. Unfortunately the cotton doesn't feel as natural as the old t-easy and feels a little more synthetic. Also it is not completely tagless - there is still a wash label on the bottom inside corner that you may want to cut off - for the slim fit shirt here there is also a black tag at the neck.
Despite the short comings of the t shirt compared to the t easy (and this actually is completely subjective - you may personally find these t shirts better and freer on the body) this still remains the best option to buy your t shirts for the gym / social or work use. Once you find your size you cant go wrong in buying 10 inoffensive t shirts for the price of one poorly constructed high street t shirt (that will generally speaking have some cliché graphic posted all over it).