At least for spellcasters. One who never uses spells can avoid them imo
If you're trying to make a character that is as powerfull as possible you're going to be a spellcaster.

Also, what Gurkog said. For characters that aren't spellcasters Atronach has no downside, just a huge 50% chance to ignore enemy spells.
I'm trying to remember the last time I wanted to create a character who was as powerful as possible (yawn.) :wink_smile:
Well yeah, that's why I put in the "if" part.

Birthsigns can be divided into three categories. The first category are attribute enchancers (Lady, Steed, Thief, Warrior). These provide an early benefit, but they are useless in the long run. This makes them a good choice for people who are not going to play their character long enough to max out their stats as well as for people who play without leveling (either never use major skills or never sleep). But if you expect to eventually reach 100 in your relevant attributes they are ultimately useless.
The second category are power providers (Lord, Lover, Ritual, Serpent, Shadow, Tower). Some provide more useful powers than other, but ultimately the effect of every one of these powers can be replicated by a spell. So they can again provide an early benefit, but they are not very useful in the long run.
The final category are the Magicka birthsigns (Apprentice, Atronach, Mage). Apprentice is clearly weaker than Mage (50 extra Magicka is not worth the 100% Weakness to Magic), but both Mage and Atronach are good birthsigns that always remain useful.
One thing I want to make clear is that I'm
not saying that people should pick Atronach or Mage because they are better than the rest. I'm just saying that
if you want to make a powerful character, there is no better choice. And of course I agree that if you care more about flavor than about stats, any birthsign can be fine.