As for drops – almost always, your defaults are the best. There are no straight upgrades (well, some can be argued); most weapon drops are designed to benefit you in one style or aspect of your game, but there’s always a trade off.
Some of the melee weapons can be straight upgrades. As medic, there’s no reason to not use an amputator over the default bonesaw, for instance.
But yeah, default loadout’s pretty good–close to ideal even–for most classes. Some items completely change the way a class is played (Eyelander + Charging Targe = demoknight, Gunslinger = battle engie, half the spy items change the class’s tactics) whereas some change ideal tactics a bit (Backburner favors sneaky pyros, phlogistinator is great for those who charge in burning, while degreaser is better for reflecting shots and switching weapons to combo with the axtinguisher / shotgun / flaregun. The crusader’s crossbow almost turns medics into a sniping class for dealing damage and healing, and the huntsman makes snipers more mid-range combat snipers.) If you can keep stocked with ammo as heavy, I recommend the sandvich over the shotgun, simply because feeding your medic is good behavior, and being able to duck back and heal anywhere is also kind of nice.