I'm putting together a kit list for hiking and the last time I went on a big hike I wore a breathable t-shirt, but I didn't find it that comfy and it also tended to start to smell quickly. Any tips on fabrics / garments for upper body when hiking long distance and multiple days?
A lot depends on the climate, on your budget, and on how much you sweat on the move.
Pros and cons of merino
Merino is the king if you want to prevent odour and most people find that it's the most pleasant against the skin. It wicks quite well but holds around 25% more water than synthetics and takes around 50% longer to dry. Another issue is abrasion - if you wear a high wool content shirt directly under a pack you will quickly get holes in the shoulders and upper back. Finally, good wool garments can be expensive - keep an eye out for sales.
Because of the advantages of wool, researchers have been working hard to offset the disadvantages through blending with synthetics and through structural innovations such as grid weaves. Two of the more interesting developments are Power Wool from Maldon Mills, and Rab's MeCo fabric from the oddly named 37.5.
Pros and cons of synthetics
Synthetics are cheaper and more durable, and wick even better - though if you sweat up in the cold (always a bad idea), there's a danger of flash chilling as moisture is drawn off your skin. Warmth for weight it is lighter than wool, and it dries a bit more quickly. But even with modern treatments most people find that it builds up a formidable whiff after a few days of wear, and it can be difficult to wash out the smell in the field.
In my experience, the best of the synthetics is a good polyamide, which out-performs the more common polyester and polypropylene on odour, durability and weight. If you don't sweat too much you should be able to wear it for quite a few days without becoming too antisocial.
A little known alternative - and my personal favourite
A little known option is to wear a technical mesh such as Brynje next to the skin capped by a lightweight wool or synthetic layer. This is essentially a sophisticated string vest and gives the best of all worlds. Although it's polypropylene it doesn't build up odour, at least in my experience, presumably because it's so well vented. In the cold, it traps a layer of warm air against your skin. When it's wet or sweaty it keeps the damp capping layer off your skin. And when it's hot, you can unzip the capping layer and dump heat instantly. It's widely used in Scandinavia, but has only recently been spreading to the UK and the US.
The odors probable came from your shirt clinging on to all the perspiration, and not having a chance to dry out. If you can afford the weight of a spare (or sleeping) shirt, spring for it so your wet clothes can dry overnight.
It also helps to coordinate the rest of your clothing. If you're sweating, make sure anything you're wearing over your baselayer will not impede the evaporation. For windy ascents, a breathable windbreaker is ideal.
I recommend cotton only as the outermost layer if you plan to be near a campfire. Synthetic materials do melt, even from stray embers. Even then, it wouldn't be of much use since you would only wear it at camp or while sleeping. Spring for wool if it's in your price range.