If you just want to level, every class works fine. If you want to solo Heroic 4 Missions or Flashpoints you need a Tank Class w/ Healer Companion or vice versa.
In my opinion the strongest solo class is the Medicine Operative. Companion Healers dont cut it in tough fights, but you as a healer can keep a geared Kaliyo alive all day long. This combined with stealth, a great amount of CC and the ability to deal damage while your HoTs are rolling is stronger than any Healer/Tank Combo out there.
Most healers dont have the time to deal damage while keeping the companion alive in tough fights, but the Operatives healing style suits that just fine.