The OP did not specify "breakthrough duties", although there was no indication whether or not the escorts would participate in the actual assault. That obviously has an effect on the "best" units for the task. The OP also did not state the size of the Assault group, nor how much money, BV, tonnage, personnel, or whatever the limiting factor might be, was available for the escort party. That doesn't exactly narrow down the choices.
Participating in the assault? The escort force might well consist of MORE Assault 'Mechs, or heavy vehicles. Not participating? Then it's probably Light or Medium 'Mechs and/or vehicles. The exact mix probably depends on terrain and the expected opposition.
In MOST situations, a Phoenix Hawk is my "go to" solution. In this case, I still find it to be a good "all around" choice, although having a faster bug 'Mech and several assorted vehicles to supplement it (possibly with a VTOL tossed into the mix) would probably be an improvement for not much extra cost.