OmniMechs make great ProtoMech transports. With the release of RS3150 we have 9 different ProtoMechs with the Mechanical Clamp System (Cecerops 5, Centaur Z, Procyon Z, Roc Z, Gorgon 5, Minotaur Z, Minotaur P2, Hobgoblin, and Hobgoblin 2).
If the ProtoMech is 9 tons or less any Mech can carry up to 2 Protos (front CT and rear CT). If it's over 9 tons, Mechs can only carry a single Proto on the front CT. If the Mech doing the carrying is an Omni it doesn't take as much of a speed penalty for carrying a Proto. The speed penalty varies depending on how much tonnage of Protos is carried.
For OmniMechs, you subtract 3 tons from the tonnage of each ProtoMech (to a min of 0) before calculating the total tonnage of Protos carried. Up to 10% of the Mech's tonnage is no speed decrease. Then 10% to 25% is either -3MP or 50% decrease (round faster) to your walk and recalculate the run. You take which ever number gets your Mech faster. Over 25% of the Mech's tonnage is a 50% walk MP (round faster). For the 50% it would mean a 7/11 Mech becomes 4/6.
Depending on what ProtoMech(s) you want to carry you get some interesting (to me) choices for best transports, it ends up being OmniMechs that are fast for their tonnage. The Phantom, Ice Ferret, Viper, Black Lanner, Gargoyle/Naga/Naga II show up a bunch. And the Cephalus, Fire Moth, and Wulfen, are just so fast in general, they show up a bunch despite the speed reductions.
I even made a spreadsheet to track what are the best options & combos.