I like polo shirts. A step up in smartness from a t-shirt without the ironing overhead that comes with a proper shirt.
Currently wearing British Racing Green ones which I like. My Fat Face ones were good quality too but they are looking dated a few years on. I have a couple of Voi ones which are quite tight fitting and already losing buttons and a Jack Wills one which is lovely quality but the collar is too big - I don't want to look like Harry Hill.
Would only consider wearing one in the absolute hottest of weather as I abhor and will not wear a plain t-shirt. Otherwise, always a proper shirt, be it long-sleeved or short. Tucked in too.
Classic polo shirts are beyond fashion but retailers insist on trying to 're-image' them with stupid designs or writing all over them to make them 'on trend'. Stick with the old school standard and you have casual with a hint of smart thrown in...YMMV though
I prefer shirts, but I do have a few polos and rugby tops for when I want to be really casual. I don't wear round necked, collarless t-shirts unless I am running. They don't suit me.