One man's opinion.
I'm going to be objective about this...at least as much as I can!
1) Erde: I've read a little about. It seems fairly interesting but I cannot honestly make a valid assessment at this point since I don't have as many TL products as I like to judge Erde.
2. Arcansis: I have looked over, at least at the site. It's nice and certainly well thought out. Reminds me a lot of a combination GH and Romansque style of running a world. Certainly one of those that is not only different BUT also has a lot going for it, especially if you like Freeport.
3. Iron Kindgoms: This one I haven't gotten into much but I do remember it being based off a series of books. Honestly I cannot say I favor a campaign with guns in it...but then again it's up to you.
4. The Scarred Lands: Now this one I say has a lot of strengths but a weak technical weaknesses too. Strengths: Complex characters, not just about good versus evil, highlights the differences in divine magics as well arcane as well, has a plethora of unique monsters, spells, magic items, and interesting places. Weakeness: Maps aren't always that good, and some times not enough of them in some supplements, no adventures for it, (YET), some of the older products have some errors (but what product is error free I ask you?), and can be a little confusing if you're not used to using more story based plots than actual number crunches.
Still I favor the Scarred Lands. It really focues on classes that, for the most part, most campaign worlds don't ulitize well enough. Plus Hollowfaust IS probably one of the more intriging cities in recent fantasy history. So I guess I'd say Scarred Lands
If you asking, have I played there? No. I've tried running it...but so far not a warm reception from my players (Course these guys don't like ANY d20 product EXCEPT Necromancer Game stuff.)