Barracks: Paladins. Pure power, awesome design, and they make me feel more safe with a tower than any other barrack. Quotes are cool, and they serve their surprisingly fun tanky purpose very well. No tower can substitute for them.
I almost picked Assassins, but they are actually easily destroyed in pretty much all of the Elite Stages, and since barracks were at their most UP in Frontiers, I can't pick them.
Archers: Rangers Hideout. All the Archer towers are great in my opinion, but these ones stand out. While Mages dealt with strong enemies and Teslas killed the weak ones, Rangers made every tower so much more powerful by slowing down enemies and killing off weaker ones. The concept of it is cool, and seeing the execution of it is even more awesome because of how quickly many foes die.
I almost picked Arcane Archers, because while Golden Longbows are good for long-range instakills, they seem too similar to Musketeers, and do far less damage at close range. I'm surprised nobody has mentioned that yet. Arcane Archers are just great, with their raw power letting tem handle things themselves, and yet are also capable of supporting towers through stunning a strong enemy and weakening magic defense. They just aren't as thematically awesome or visually appealing as Rangers.
Mages: Archmage. The fact that I never buy Twister and still want this tower on nearly every stage shows how good it is. Critical Mass is a nice, cheap way to get better damage, though the damage bonus is a bit minimal until the final upgrade. Charging up 3 bolts at once was one of the things that made me so excited for Frontiers in the 1st place.
I almost chose Wild Magus, because while HEMs deal good damage, Sentinels are too expensive for the low damage bonus, even with the extra range. Wild Magus also has an Instakill that damages other nearby foes A LOT, often killing weaker ones in the process. The bonus over time is great, too, but not as life-saving as the Archmage.
Artillery: Tesla. I would not have beat Pandaemonium without you, Tesla! People say it's OP, but I can't find a way to beat cartain levels without it. I think it's balanced, just necessary to be used very often in levels. The AOE is so cool and satisfying to hear, along with wrecking many types of enemies, and the lightning beam is also really great. I love how this tower can handle itself so well without needing support, though having a Rangers Hideout and Paladins with a Tesla is nearly unbeatable.
I almost picked the DWAARP, because I love towers with a full-range AOE, but it is unfortunately too expensive.