LogFAQs > #965835432

Putting aside YugiNoob's irrational hatred of tanks, of the ones you don't have there Stronghold stays good the longest (especially since you need 5 teams eventually), but can be farmed from a side story. Once we get more SSR defenders he gets phased out.
Kyle is good for now, but drops off fast. All he's really good at is building walls. There aren't many situations where he's a better choice than other rangers. He is listed as SS for Shadow Palace (which we don't have yet), but I can't honestly remember why. Maybe it's a part after I quit.
Titan looks cool and I love seeing him stomping across the battlefield, but costs too much to field (he cost the same as an Awakened, but is nowhere near as good) and his gimmick of getting stronger when allies die isn't particularly helpful.

It mostly depends on what you need more, another defender or another ranger. Another thing that should be taken into account is that I never got Stronghold in SEA, so my recommendation on him is based more on theory and the info on Prydwen. YugiNoob should grab Titan since he already grabbed Kyle.