Depends on the mechs/Armor being supported. For the big slow stuff, I find mechinf to make very good escorts, able to keep up with your armored stuff a lot more easily than you might expect, and able to respond to close attackers coming from any direction.
For faster cavalry units, I'd go for heavily armored foot troops in fast hover or VTOL APCs. They can dash forward and quickly secure a strategic bit of dirt, allowing you to run some hammer and anvil action.
In heavily wooded or hilly areas, go for jump troops. They can navigate the terrain even faster than many non-jumping mechs and can be persistent harassers, keeping enemy mechs on the defensive lest they endure repeated backshots or kneecap attempts.
In cities, nothing beats motor infantry.