For me it's Iresso. With the lvl 40 Trooper pvp set (upgraded with 45-49 mods), he is enough to complete the 5 normal daily quests on Ilum in a moderate time. I also tried the [Group 2+] daily but the Sith Lords just destroyed us.
Why not someone else?
Qyzen's attack delay, the patheticly low range on charge and the lack of a custom Techblade are REALLY annoying, if all issues were fixed I would use him just because he's a melee tank and I like to use Forcequake.
Nadia is awesome for lower level enemies (3-4 below you), but everything on Ilum (for example) just obliterates her. While leveling, her aoe and my Forcequake took out most standard enemies in a 1x Strong, 2x Standard group and her damage was enough to hold the Strong.
But now, even with pre-combat shielding, by the time my first forcequake is finished her health is in the 10-20% zone and the standard enermies aren't even dead.
Also, she is rather useless against most elites, she can't generate enough threat with her damage against my two dots and healing in longer fights.
All the other companions are rather useless for a Seer (in my opinion).