Alistair can become the best tank you have (Shale is pretty damn awesome though) but Alistair needs a LOT of delicate work with his stats and abilities to be fully effective. I suggest pumping his strength to use the Dragon Armor (if you have the DLC) ASAP because once he has that he starts to be a beast. Then you can focus on his CON.
Might check out some of the mods being created, specifically the mod that stops the auto level on your NPC as soon as they join the party, allowing you to have full control over their abilities from the start.
There is also a mod that eliminates the preset abilities your main character gets when created (specified by the path you choose).
And of course there are the complete respec mods that will clear all in a click.
Going for my 3rd play through, I like my mage but I feel the mage as a main is overkill when you have Morrigan and Wynne picked up early in the game and can customize them to your liking with mods.
I loved the rogue my first play through, going to go that route again but with a human, and on nightmare (with nightmare plus mod) and a couple of other mods that make the game play different.
I love mods, I love the modding community.
http://www.dragonagenexus.com/downloads/categories.php