PCC Pinpoint. He's like a Powermaster and a Targetmaster while having one of the best bot modes of any minicon. BW Neo Guiledart, because 3rd mode is dead triceratops.
Since most are (thankfully) thinking beyond the original 6... Doubledealer and G2 Dreadwing are my 2 favorites.
Blitzwing. We need a Classics Blitzwing figure already. I also hated how his storyline was basically given to Octane in Season 3. But then again, Octane's toys needed to sell.
+1 on that mind you doubledealer rocks as well both ways. (i know he's not a triple changer but he really is)
G1 Blitzwing for me, no question. He was my first as a kid, and he's still my absolute favorite. Coincidentally, Animated Blitzwing is up there for me too.
Are we talking characters or toys? G1 Blitzwing (all appearances except Triple Takeover), was always my favourite Triple Changer character. Especially the episodes of season 3 where they depicted and referred to him as Octane, but referenced his (Blitzwing)'s past deeds and reacted as if he was Blitzwing. If we're talking toys, I'd probably have to say Warbot Defender.