Almost literally anything will do the job, so look more at guns. You want something with either as little gun as possible and a lot of speed so that it can quickly deploy the Mads for cheap, allowing you to afford more Mads in your force...or something that's slower that can carry them when needed, but is more focused on fighting alongside the suits once they are deployed. As with most infantry units, Marauders are lacking in range and concentrated single-slug punch, so pairing them with a machine carrying PPCs and big autocannons or the occasional Gauss will allow you to open big holes that the suits can exploit once they're in firing range. Things that put said big guns in the arms or centerline are better, because the guns aren't covered up while the suits are mounted. Consider things like Panthers, Enforcers, Warhammers, and, well, Marauders. I like Cataphracts for my VI Legio groups, though their ranged power is partially cut when the torso cannon is covered.
Wether fast or slow, try to use units whose base Walk/Cruise is an even number. So long as it's not a 10/15, the -1 MP for carrying the battlesuits isn't likely to drop your TMM below what it would have been originally.
If you're looking at something fast to quickly deploy your Mads ASAP, consider the Heavy Hover APC. It's dirt cheap, widely available, and in addition to carrying Mads on the outside, you can also carry a powerful infantry contingent on the inside as well, such as a squad of Ravagers, or a full half-Century of heavy foot troops. Five APCs carrying stuff like that means a VERY powerful(if highly specialized) force for surprisingly low BV.
