Realistically, bipedal tanks are too unstable, expensive and too big of a target to be rendered feasible (compared to others). They’re also immobilised a lot easier than, say, treads.
Spiders would suffer similar issues to bipedal, but they’d at least be more stable and possibly smaller. Also, they’re not as easily immobilised. Also more of a feasible substitute.
I believe the reason we use treads is because they’re great for all-terrain movement and can support the tank’s weight better. Also, they don’t sink as easily as isolated wheels.
Speaking of wheels, they’re a feasible substitute to treads, albeit a bit less suited for certain terrains (e.g mud stuff)
Repulsors, if you could figure out the tech, may be a feasible substitute, due to not really being able to sink, as well as not being much of a main target. Would be more expensive, but probably worth it.
Combinations depends on the combination. Treads and wheels would just be odd, since treads have wheels.
A walker with wheels would probably be a good idea, in case the legs are blown off or whatever. Sort of a backup form of movement.
A tank that could turn into a spider walker at will could be a good idea.
I don’t think repulsors really need a backup, but if they do, treads or spider legs.
Bipedal walkers are silly, but at least they look cool.
Spider walkers could be good, but a bit costly
Treads are the most realistic
Wheels are a good substitute, but a bit less reliable off-road.
Repulsors are a bit ahead, but may be good for movement. Sucks when you get EMP’d. Probably.