1. Must be made of quality materials
If you're looking for comfort, then the chair must contain quality materials. The seat structure must be made from durable materials and fine to the touch like stainless steel or titanium. The upholstery must be made of natural leather. Avoid synthetic leather: it may be discolored due to dye used, usually has an unpleasant odor and it has a cold feeling.
2. To be resistant
I think you want to use the seat many years from now. If so, then you must choose a chair made of resistant materials. Like i said at point 1, the perfect materials are: stainless steel, titanium and natural leather upholstery.
3. The seat must be adjustable
Your future office chair shall be adjustable in height and also it must have an adjustable backrest. With these options, you can be sure that it will fit perfectly on your desk.
4. Test it!
If you can, go to a furniture store and test the seat before buying. By testing the seat in shop, you avoid to buy a chair that is uncomfortable. In this way, you can compare many models and choose the right one. You can also read the reviews online. Most reliable are the reviews on major sales sites or on furniture blogs where people can comment and leave impressions.
5. A good value for price
Make sure you pay the right price for the chair. See what are the best features of the seat and evaluate the price. After doing these things, see what chair meet your needs and is consistent with your wallet.