A few comments... there is no really "best" antenna for DX work.. it all depends... if the person you are trying to reach is using horizontal polarization, and you're vertical, you will have cross polarization losses, and vice versa.
Next, the A99 and Imax aren't exactly known for being great antennas, they have issues with common mode currents, and unless you get them up in the air quite a bit, have high ground losses due to the fact they they are a "no ground plane" design.
However, that doesn't mean that you won't have decent results with them...once again it all depends on what you are trying to do.
For example, let's say you have an A99 and a Maco V5/8 groundplane both at the same height.
You have the same feedline fed to both and for all intensive purposes they are identical setups connected through an A/B switch to the radio.
You are talking to a station 650 miles away and you switch back and forth and notice the A99 works BETTER than the V5/8 even though the V5/8 is a true ground plane antenna and is a 5/8 wave vs a 1/2 wave design.
Well, it does make sense because the A99 has a higher take off angle which is normally undesired property, which means it has less energy out toward the horizon and more up toward the sky that gets wasted. But for this relatively short "dx" contact, the A99 is ideal.
Now, same frequency, same situation, and a station from 2500mi away tells you that the V 5/8 performs better. Once again, makes sense, a lower radiation angle.
A word of advice is try to avoid the people who tell you one antenna is better than another due to the fact that they talked to "so and so" and make "x amount of contacts" to where ever on it. These results are shaky at best, mentally biased, and remember even a piece of wire strung up between trees with the right conditions can permit DX contacts.
If I had to have ONE vertical antenna it would be either the I-10K or the V 5/8 or some copy of it. I would avoid the A99, Imax 2000, or the Sigma 4 copies.
My recommendations are based on technical facts and efficiency and also radiation patterns.