GDI - Patch
I use patch for when i go in the tunnels - hes EXTREMELY GOOD against buildings (i can see 4 Patches destroying a building by themselves) so its a quick way to get points if your loosing. Hes also very good against Infantry (one i killed Raveshaw, a laser chaingunner and nearly a chemical warrior till i died) if you know how to aim well. PLUS, he is one of the units that can see stealth soldier better!

Nod - Stealth
I use this guy alot on base defenseess maps, its easy to sneak in to the enemy base. Just keep away from people and you should be ok.
A good tactic is to plant C4 into another buidling and then go to the buidling your after - wait nutil it says XXX is under attack, then plant your beacon. Engys will have to either choose between reparing the other buidling or defusing your beacon. Either way, itll help your team, beacon fails, building still damaged, beacon succeds, buidling destroyed