Which weapons are considered "upgrades" in TF2?

Assuming "upgrade" means "has no downsides and additional non-cosmetic upsides", the current list would be this:

• Third Degree (same as Fire Axe, but also damages everything else connected by medigun beams)
• Hotrod (same as other hats, but due to a bug, if a Spy's disguise is wearing one and he's holding a Sapper, it'll flip over his face and expose him to attentive players)

There are not currently any other pure upgrades in the game - any weapons that seem so are either purely cosmetic differences (Golden Wrench's Australiumization effect), replace a weapon with a support item of uncomparably different functionality (Sandvich, Gunboats), or have an upside that significantly outclasses the downside (Amputator having a healing taunt is much more important than the 20% damage reduction, which while probably technically an "upgrade" I don't think is the purpose of this question).

Nearly all weapons that are not "default" either have a penalty attached to them, or change drastically the nature of that weapon slot. So items that are nothing but an 'upgrade' are very rare.

Here are, as far as I know, the items that exclusively add to a "stock" item's ability, with no downside or change in functionality.

• Fire Axe > Third Degree - Adds "shared damage" between targets attached by medic beam.
• Bonesaw > Amputator - Adds healing taunt that heals multiple team members at once.
• Fists > Holiday Punch - Causes target to laugh when struck from the back (stunning effect)
• Bat > Fish? - Special Kill Tracking (You might consider this an upgrade, or you might consider it a downgrade that reveals your location)

Note: Amputator and Holiday Punch both have downsides. The Amputator does -20% damage, and the Holiday Punch does no crit damage.

There are only 2 direct upgrades in the game.

The Third Degree: With no downsides, the only upside is that this weapon hits enemies that are connected through a Medi-Beam.

The Solem Vow: With no downsides, the only upside is that this weapon shows you the HP of enemies that you are aiming at.

Weapons such as Ali Babas Wee Booties, Targe n' Charge, Kritzkrieg, RazorBack, ect, ect are side grades. Even though they have no downsides (stat wise), they do take up a slot. The Booties take place of your Grenade Launcher, and the shields take up your sticky launcher. Same with the Sniper Backpacks, Soldier Boots, and Heavy Food. The Kritzkrieg (with no down stats) is a side grade as well. It turns your invulnerability into insta crits. The equalizer/escape plan are great melee weapons, but they prevent being healed (and the escape plan makes it so you take Mini-Crits while out), so they are not upgrades.

I hope this cleared up the above answers.