The only character that is kind of a "must have" is Wynne ... you can build Morrigan to be a competent healer - but Wynne will almost always be a superior healer...
The other party members can be any one of dozens of different combinations built for different styles... but a healer is always an integral part of an optimal fighting unit- and Wynne is simply the best pre-built healing party member to have.
You can find her in the Circle Tower.
As far as "best in class" I'd say: the best Tanking character would be Alistair the best DPS Warrior would probably be Sten or Shale (Oghren shows up much later allowing for less early stat building, and the Dog has few talents) the best rogue would be Zev (Leliana is better for ranged DPS, but Zev's melee dps > Lel's ranged dps) the best DPS mage would be Morrigan the best healing mage would be Wynne
All that said - you can build these characters to be other styles and builds - but considering for optimal builds and thus utilizing their beginning talents - these are the classes and styles they seem to be designed for. Also - the player character's will almost always end up a superior version of whatever class and build it is relative to the analogous party member.
Scale up an atomic nucleus to a marble's size - it's electron is thus orbiting 3 miles away. Meaning, the closer you look at yourself, the more you disappear.
me (DW warrior champ/zerker) allistair - tank (templar/champ) Wynne - healer (spirit healer) Leliana - support (bard/ranger)
my character did a majority of the damage, allistair did fantastic as a tank and wynne kept us alive extremely well. Leliana was probably the weak link in the chain, but she opened the locks, played buff songs and had an animal out for support so she was still worth the slot over another more damaging character. plus having a third melee person up there would make things more cluttered
I also am playing as a Human Noble Warrior. Usually my party has been Wynne(sp?) for healing, Morrigan for attack spells and Leliana(sp?) for ranged attacks(archer) and her lockpick skills for chests. Sometimes i switch out Morrigan for Shale for the extra tank.
I've put most of my mains skills in sword and shield so i'm a pretty tough tank myself. Having 2 tanks, a healer and a ranged rogue works pretty darn swell for me.
Gamertag ~ Dharma Troy o_O "There's only one man who would dare give me the raspberry jam......LONESTAR!!!!"