I'd think you'd want to take Ossyan or Ravyn as i feel both really want Aiyana and Holt more than anything to help secure the Menoth match up.
It's a difficult choice depending on whether you feel the accuracy and snipe wins out over the damage and more toolboxy approach of Ossyan (also shatterstorm is good for EAsphyxious and Morvana)
I'm not sure which i prefer because they both behave differently but do shooting really well. Ravyn is probably better for the simple fact that snipe is incredible on MHSF, that being said though with the possiblity of skewed double colossal lists Ossyans feat becomes a lot more attractive in addition to its ability to flatten stealth medium infantry (via sfa).
I think its a difficult choice at least anyway.
I love Rahn for a three caster tournament just because he is so good at a lot of scenario's (particularly the flag ones) and his list doesn't have to be that magic dependant to still funciton effectively. A single unit of mages does pretty well and two is also really good but it doesn't really need any more than that and if the rest of the list combined arms then the mages can flex where needed because they aren't a bad melee option when it comes to it. Not stellar but passable.
By all accounts if Bethayne can run well without hex hunters then Rahn should be able to run k with out over balancing the lsit with mages.
I really like Kaelyssa for three casters too - as has been stated above she has a lot of answers and the double sylas rift node isn't always apparent to all - not to mention a great way to clear out stealth and if you couple it with an AFG you can really make the ground tricky to get through. I like her with a pretty balanced list since she's a relatively balanced caster - i haven't played her in a while though so maybe that would change now.
I also like Pvyros too - mostly because i like him, i feel though that he has strong answers for Legion (particularly ELylith) and the simple thing is that your heavies running 16 are engaging feated ravagores and ravagores under Lyl aren't the best combat monsters. I had a list a while ago designed for this match up since she was really popular in my area and it did really well (until i scenario'd myself - stupid grind!

). Anyway I like Vyros and depending on whoelse i took he'd be a consideration - it also helps that he isn't popular and a lot of people have relatively little idea what he is capable of.
If i had to pick today i'd take Rahn, Kaelyssa, and Ossyan. Not sure for Gencon though - but with possibly four of the colossals present i'd probably rather have Ossyan than Ravyn - although caster kill is still a pretty big trump of hers. Maybe Ravyn!