Here's a slightly different take.
Warriors are the easiest to learn, they play extremely well with a team, and they have the highest sustained DPS from plain auto-attacks. Rogues are a bit more difficult because they depend more on positioning, tactics and ability use than warriors, but they are also more versatile and fun (stealth!). Mages are the most powerful by far but also the most difficult to learn, and they don't play really well with teams most of the time, except as support.
A first playthrough is probably easiest with a defence-oriented sword & shield tank (strength and dexterity favoured about equally). They are the least complicated of all, and since it is the first time you'll have to learn the game as well and a bit of every other class in addition to your own. They have great survivability, and they lend themselves naturally to leading from the front and 'titanium speartip' tactics. At later levels they get so powerful that they can solo groups of ogres without breaking a sweat.
A good sequence of classes is warrior first, then rogue, then mage. After a rogue, any warrior feels a bit pedestrian. After a mage, everything else feels horribly pedestrian, although a stealthy DW rogue is still plenty fun despite that.
My first rogue was a DEX dual-wielder with daggers, and she is still one of my favourites. Final stats: 20 strength with items, 30 cunning with items (22 base), rest into dexterity. She was my second character overall but the first - a dwarven warrior - died at level 10 when he picked a fight with some big bad boys in the Brecilian Forest, so he doesn't really count.
Combat Movement makes positioning for backstabs easier, and Coup de Grâce turns normal hits against incapacitated opponents into backstabs. The whole Whirlwind line of abilities is so good that I gave it even to my warrior - a sword & shield Reaver(2)/Champion(4) - when she turned twenty and had already perfected all her other abilities.
Mages are great as support for a warrior or rogue Warden - buffing, healing, crowd-control, freezing clusters of enemies for quick disposal by shattering, taking out enemy mages and dispelling hostile magic. The rest of the time they can fire their staves at anything that moves without having to be told, if you set their behaviour to "Ranged".
Things are less golden for mage Wardens. If they play support for a team (eminence grise) then the focus of battles is mostly elsewhere. If they want to hold centre stage as battle mages then they get held back by any friendlies hopping around in the battle zone.
Mage + team = micromanagement. A few encounters are easier to tackle with a team but most of the time it is less hassle if they hang back or stay at home.
Against single enemies, caster mages have the lowest sustained DPS of all classes, and even their shatter abilities are not 100% effective where those of the other classes work without fail. However, they excel at crowd control and peak DPS, and usually they can project those DPS on whole groups of enemies simultaneously. A mage can dispatch clusters of enemies just as easily as any single enemy, but the converse also holds: any single enemy requires just as much work as a whole group. Hence, the effectiveness of a battle mage hinges on their ability to herd, channel, and contain the enemy hordes for maximum effectiveness of AoE spells. It's all about positioning, timing and resilient tactics (i.e. not getting stumped by spell failure, and having alternative moves ready).
Do it right and you can dispatch most enemies faster on your own than a whole team of 'physical' fighters can, without needing any potions except a few during some mass battles and some boss fights. Do it wrong and it will take forever - team or no team - and you'll go through stacks of potions like there's no tomorrow. Steep learning curve. My first High Dragon was a drawn-out, epic battle that took about 50 potions of all kinds. The last one was toasted with one lesser health pot and one lesser lyrium (working in a team with a golem, a healer and a rogue).
A battle mage's bane are groups of fire-proof elites like ash wraiths and drakes, and melee bosses like alpha ogres and revenants. They require special care and tactics, even the odd bit of kiting; everyone else gets pwned and waltzed over.
Viability at very low levels depends on picking spells for synergy, so that you can cover all tactical necessities with the few spells you are allowed to take. Fireball plus Mind Blast works well, so does Cone of Cold plus Walking Bomb. The latter is slower but easier than playing with fire; combat tactics are completely different. Or you can ride on the coat tails of team mates all the way from the Harrowing to the endgame, after slaying five wisps and one wolf on your own.
Stats allocation is the easiest of all classes: all points into magic. Lowish mana and hitpoints can be compensated with potions and items, insufficient spellpower and damage cannot. I posted a soloable spellbook a while ago; the containing thread discusses the merits of various spells and combos.