It seems to me that it is easier to design a tank for a specialist role like fire support while most 'mech designs of medium and above weight class tend toward "generalist" design philosophy. i.e all around armor protection, weapons of different overlapping ranges, et cetera.
The reason tanks are a bit better in a specialist whole stems also to the fact they are pigeon holed into it. Tanks have to pay nothing for additional heat cost due to ammo weapons, but will pay through the nose for energy boats because they have no access to double heat sinks.
Defensively
Said it before Ill say it again, Gurteltier. Rommel's and Patton's ain't too bad either, won't stand up against assault mechs but unoptimized Heavies, Most Medium and Lights.
That can be said of any tank. Neither the Rommel or the Patton are going to overpower any mech of similar weight, but you have respect a Gauss or a Ultra-10 for what they can do. If you measure up their BV to comparable value to mechs in the same range (1100-1200 BV) they stack up favorably. They both have a modest amount of armor, which when incapacitated, will allow them to fight on.
The Gurteltier on the other hand is a beast. Sure it doesn't have as much firepower as an assault mech of the same weight. What it does have though is the ability to stand its ground and delivery accurate fire either because of the TC and/or the C3 network it will be a part of. Its loss in firepower is made up for in armor protection per location. While you can disable the thing with motive hits it really doesn't matter. It is an armored brick on pretty much any side you decide to shoot it from.
Offensively
I agree that the Epona is another candidate, as well as the Pegasus Hovertank.
I don't know that I would use the Pegasus for anything other than for scouting or a tank killer. It is a bit light to do much anything else. I am also probably the minority, but the Epona hasn't aged well and I certainly wouldn't use it often in an offensive capacity except for the A as fire support. It has poor amour for a 50 ton tank, even a hover, which relies on speed to not be hit. Firepower is still decent for a hover. For offense I would rather use a Joust or to a lesser extent, the Enyo. Both are slower compared to the Epona, but put up respectable speed, firepower, and armor to survive. The Epona does only 2 of those 3 things. The notable exception to this would be what Kovax mentioned.
There are numerous intro-tech hovers that compare favorably against 'Mechs in open, swampy, or river-intensive terrain. Throw in enough trees and hills, and the 'Mechs are clearly superior. Add in advanced tech and specialty ammo, and the hovers are toast.
Unless the fight goes below water, a hover is going to be better on water than a mech. At least until it gets hit. Mechs, however outside of UMUs, generally get bogged down and deserve to die if they are foolish enough to try and fight in water against a hover. Conversely any obstacle is going to make it worse for a vehicle to maneuver, especially hills. It doesn't take much to disable a tank assuming you hit it. LB-X, massed missiles, and inferno/TC. All of them spell trouble for tanks and especially hover tanks which are more likely to take a motive, if not, incapacitating hit.