Properly fitting cycling clothes.
Yes, strictly speaking, they're not necessary for riding. I've seen people ride centuries in blue jeans. However, I've ridden in both, and it's more enjoyable to ride in:
- cycling bibs with chamois - no waistband digging into my waist, no tugging waistband up during a ride, no flapping material, no seams between my butt and the saddle, a chamois and material that move with my body with nothing to get in the way, ride a nice saddle with no padding (Brooks), or minimal padding (SI Flite or equivalent) to avoid chafing;
- cycling jersey with rear pockets - no flapping material chafing my nipples, wicks away sweat which dries in the breeze, pockets to hold necessities (phone, ID/$$, hankie, cleat covers, snack) readily available without having to stop, zipper for adjusting ventilation;
- close fitting base layer, wicks away sweat, feels nice against skin, added protection against UV;
- wool socks (all temps), keep feet drier and warm (in cold temps), don't bunch up, don't get hot spots that I sometimes get with socks made from 100% synthetic materials; available in various weights for different temps;
- cycling shoes, stiff sole to prevent tired arches and pain in ball of foot, wide enough to not pinch toes.
Yes, I'm a MAMIL (actually, maybe I've "graduated" to OMIL?), and I look a little silly, but for rides of several hours putting out a lot of effort it is far more important to be wearing functional clothes than to avoid looking like a dork.