The second thing you want to think about is melee / range balance. You need some characters (and at least one defender) who can stand on the front line of the party and go toe to toe with the enemy. Not having this means you forfeit your ability to control which of your characters your enemies attack. Similarly, if you overload on melee characters the tactical terrain will cramp up and you also won't have the ability to focus fire so effectively or to deal with primarily ranged opponents.
As a rule of thumb, you want slightly more than half your characters to be primarily melee, or at least able to stand on the front line. In a five character party, three melee and two ranged is ideal. Personally, I like 2 defender parties, because you have a really strong front line. (You can fudge this in all kinds of ways, though: for instance, infernal warlocks and protector druids do fine as a third frontline character; shamans and beastmaster rangers add another character to the front line.)
After that, it is really about much more detailed synergies.
This seems the most important party balance feature to IME.
With 5 you want 3 people who are happiest in melee & one who can cope.
I think controllers are optional though with 5 you probably want either 2 defenders or a defender & a controller (not a sorcerer - he is DPS).
A leader makes things go smoother - healing is most useful in very hard fights, often when you have a run of bad luck. If you have 2 they ought to bring some buffing or DPS along too - too heal specced laser clerics would be unhelpful. Some leaders do benefit more if the party is constituted in particular ways - Tactical warlords that use commanders strike want someone with a good basic attack.
Wardens & Fighters are the tankiest fighers especially the busted Battleragers, you need less tankiness though if you have more defenders or other robust melee types (eg Avangers, some Barbarians)
Strikers are the bestest - they make the fights finish soonest & all types but some warlolcks do their job.
So I would take
Defender
Defender or controller
Melee striker or leader
Ranged leader or striker
Someone else. I would want damage buffs or damage from this guy but any role could do this.
Really with only a druid we do not have much to work with (especially me who has not figured out druids...) but if you had 3 characters that people were keen to use people could provide good guidance on how to fill out the rest of the party.
For what little it is worth I did decide after PHB2 came out that my ideal party would be:-
Fighter or Warden (Defensive)
Tactical Warlord
Battle Cleric
Barbarian
Sorcerer
This has 3-4 solid meleers Good targets for Tactical Warlord buffs. Good healing/buffing form the cleric. Good single target & AOE DPS.
It does not have a druid though

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