I'm honestly not entirely certain if you're required to post one answer for these questions per day or not, but that's how I've chosen to interpret it. So. Since I've missed a day (this is probably going to be happening a lot), I'll do Day 2 today.
My OTP would definitely be Ike/Soren (I don't even need to think twice about this). Ok, so this might not be a terribly original choice, but I've never been concerned in the slightest with originality when it came to shipping, so I honestly don't think it matters.
I'm not sure I could ever list all the things I like about this pairing, so I'll just rant freely about it. First of all, I always feel a little sad whenever someone says they dislike the ship because of the seme/uke-dynamic, because while I'm sure there are some fan fics of less-than-ideal quality which portray them as such, I believe that Ike/Soren completely ignores this cliché when written well and IC. Both of them are very strong characters, for entirely different reasons (Ike is more of the stoic kind of strong, knowing how and when to move on instead of dwelling on the past and angsting about it; Soren, on the other hand, is strong because of how much he's managed to survive without going completely insane), so it goes without saying that neither of them would make any sense as an annoying weepy uke.
Soren can get emotional when with Ike, to the point that he cries in front of him. Twice, even. However, this is obviously not a common occurrence - he's telling the only person he's ever trusted either about issues he's been struggling with for a long time (being branded) or one of the most important moments of his life (meeting Ike) - Soren crying is so surprising because it's rare. I can't picture him crying over small things. The two times it happens in the games are not the norm; they're the exceptions.
One of the main reasons I'm attracted to this pairing is definitely because of Soren's co-dependency issues. It might sound a bit odd, because it's not something I'd normally consider romantic, but what surprises me the most is how Ike reacts to this. He doesn't reject Soren, or even find his constant clinginess and need for approval annoying. He's actually fully aware of it (surprising, since most characters in fiction or even real life would either take advantage of it or act oblivious to it), and he accepts it. In PoR, he's constantly reassuring Soren that he needs him and that he likes him the way he is - one of the best examples of this is the scene where he apologizes for having made an offhand remark about Soren's rudeness, because he knows that Soren isn't taking it too well in spite of trying to hide it:
Ike: Soren... About what I said this morning...
Soren: Yes?
Ike: About the way you phrase things.
Soren: I... I ought to apologize for that.
Ike: No, don't apologize. I know you. I know it's been bothering you, hasn't
it?
Soren: No. Well...
Ike: Don't take it personally. I'm no better, you know. Your ability to speak
plainly the things others won't is part of what makes you brilliant.
Others are too bound by courtesy... With you, I trust that what you say
is exactly what you think.
Soren: Well...yes... Thank you, Ike.
In many cases, this kind of co-dependency would be more of a curse than anything else. But since Ike is actually reaffirmative and supportive of Soren, their relationship would end up working out really well. (Actually, I'm kind of shocked the game handles this so well, but I'm glad it does.)
I think I was also surprised by the excellent chemistry between Ike and Soren, partly because their friendship is somewhat unexpected. While they share each others' bluntness and snarkiness (I love both of these traits by the way ♥), most of their other characteristics are rather different. Most notably, Ike is generally a nice casual kind of guy who does the right thing and has no problems with talking to others, whereas Soren is a loner, very pragmatic, and usually a complete jerk to everyone but Ike because he feels he doesn't need anyone else. In spite of this, they're great friends and feel at ease around each other, in a way I wouldn't say either of them does around anyone else. And while it's true that Soren puts Ike up on a pedestal and favors him over practically everyone else, Ike shows some signs of favoritism as well (for example, notice how he doesn't want Shinon or Jill acting racist but is completely fine with - or just doesn't comment on - Soren's extreme dislike of laguz).
While I wouldn't necessarily say that the game plainly shows that Ike has romantic affections for Soren (because, imo, it would definitely be possible, or even quite easy, to make an argument for Ike being asexual although I could never really see him being straight), the opposite, that he doesn't, isn't as easy to prove - though it almost never is. I do think he's closer to Soren than to anybody else, however, so there's that.
The single best argument for Soren liking Ike, in my opinion, wouldn't be their unlockable base conversation in RD. Instead, I'd say the conversation with Aimee in 3-2 proves this - because it would honestly be hard to interpret Soren interrupting Ike right before he tells Aimme that she's "beautiful" as anything other than jealousy. Normally, you'd expect the calm and rational Soren to be all for free weapons and supplies, but nope! Not if it requires Ike to tell a woman she's pretty. (A-and I honestly find his jealousy kind of adorable~)
... yeah, I've already spent more than an hour on overanalyzing the dialogue in a video game to pieces. I should probably stop here. And go to bed because it's really late. (But it feels really nice to rant about this! I've always wanted to say something about it, so.)
In other news, my fandom list thingy still isn't done, and I'm not sure if it ever will be, so I'll just randomly update it every now and then. Also, I need some kind of Ike/Soren icon. I should go on a fan art-hunt.