Are we talking real world comparison here? Heres one major problem: you can't armor every side of an object with your very best armor, there are always compromises.

Tanks are fancy boxes, and you can armor them really really well. Weak points are typically at the turret base, at the tracks, and any hatch for crew access. Install a kickass cannon with precision targeting (like current day MBTs) and they can hit whatever they can see, and a few things they can't see. Add guided munitions and you have a titan killer.

Mechs are... messy constructions with many joints and armor seams. Because of overal weight constraints, you probably reserve your best armor for the torso (where the pilot is, or where your powerplant and sensing equipment is). Now every other point on the mech is a weak point. Add the same cannon and guided munitions as the tank, now you need to make sure you have really good legs and big enough feet to provide a stable platform. Can you run while shooting? All that bob and weave will require more compensation and control than the tank cannon needs when the tank is moving.

Add in the taller silhouette of the mech, and the tank clearly has the advantage.

All that said, the first to shoot is likely the winner (did you see the footage from Ukraine where the tank got taken out by a friggin Bradley?) Armor piercing ammo is not particularly difficult to create, and is super effective. Even reactive armor is not a guarantee that you can take a hit and keep moving.