The original is the one with vertical slots to see the glass inside. That's what I have, three of them. Seems they are being phased out in favor of the new v2 with the open glass look. Both are 4ml. To my knowledge, the 1.8 and 2.5s are new to the scene, or at least I never saw them until recently, about the time the V2 mega appeared.
All three with the window bars are the prior model. The mini is 1.8, the ego one is 2.5, and the mega vt is 4.0. Those are now replaced vy the mega v2, 4.0, and the v2 xl, 2.0. They all take the same coils, whether the so-so cl series, or the newer, wonderful CLRs, which come in both kanthal 1.0 ohms, 0.50 ohms, and the Ti tc.
Gets confusing without a scorecard.
The Tron-s replaced the Tron-t. Seems nearly identical to the mega, but for the window and the semi-hidden air flow slots. Takes the same coils.