When Brutus refers to Cassius and Titinius as “the last of all the Romans,” what he means is that they were among the last few noblemen fighting to uphold the time-honored principals of the Roman Republic: that the highest honor and was attained from serving the state, and that no individual could attain a position of unchallenged power. 

Brutus seems to be bemoaning the fact that of the sixty or so conspirators who had joined together in an attempt to return Rome to its former virtuous state, there is now only himself and a few followers continuing the battle.