As a fairly frequent flyer I feel qualified to preach my views on comfortable flying and having just got back from Valencia I thought I'd share with you my favourite travel outfit. For me comfort comes first but equally I don't want to rock up to the airport in sweats so I find patterned trousers a great compromise. These ones were around £10 from New Look so nothing special but are so comfortable compared to jeans. As for shoes I'd generally say wear your heaviest pair as that makes for more room in your luggage especially if you are only taking hand luggage like I tend to do. I don't think your top half is of much consequence as you'll be sitting for most of the journey, however if you're not comfortable in crop tops then avoid wearing one.
I'd always recommend taking a hoodie on a flight as the air conditioning can get really cold and if you want to sleep it can make a great pillow. If you're wearing sandals like I was then take socks! It's such a lovely feeling being able to take your shoes off and put them on- instantly more comfortable and cosy. Another obvious must is iPod and a book so you don't get really bored. If your skin is easily effected on flights definitely take a moisturiser. I don't tend to take my makeup off on flights unless its a long one so a hand moisturiser will usually do. Also don't forget that any liquids you take in your hand luggage must be under 100mls. If you're unsure on what you can and can't take in hand luggage check out this table on Hand Luggage Restrictions. Finally don't forget your passport!!