Warlock, by a long shot. I will never play any other arcane caster. No paperwork on remembering spell points, spell slots used, spells prepared, spells memorized, where is my spell book, did I have time to prepare spells, etc. Just get up and go! And of course, they are the most stylish class ever produced by Wotc or Tsr.
I think Hexblade, Swashbuckler, Soulknife have style, but there is something missing, can't put my finger on it, in each.
And I think all 4 would lose power, esp. at high levels, but I am not sure that is a bad thing.
Maybe a campaign that just had classes that lose power at high levels? Then there would be no relative loss...
The psion is great for me. Especially the Shaper sub-class. Mind you, none of my Dm's are crazy about psionics...
The Scout seems nice. I would play that before I played a ranger (I think the spells get in the way of my concept of that niche, so it is nice to avoid them).
I have never read the Artificier or Marshal or Healer, so can't comment. The word of mouth is fairly positive, although I have heard that the Healer is best for "casual gamers" who don't want to get too involved, but like to do something that others value. I wouldn't want to play a Healer, but would love it if others did in my party.
I prefer the Spirit Shaman to the cleric, but again, I would like a Warlockized divine caster (I am spoiled now). But that would need more playtesting, and limits of some kind produced on the healing.