The Mammoth Tank is the best steamrolling unit. Its cannons (railguns once upgraded) can easily shred through vehicles and infantry alike. Along with its 2 SAM launchers and Anti-armor missiles can counter any opponent counterattack.
The MARV is also effective, but it does not do a huge amount damage with its tri cannons because it is meant for turning Red Zones into Blue Zones.
The GDI has the newest Mastodon walker and the newest Mammoth Tank for steamrolling purposes.