Since they pretty much all have the same energy density, it will be the physically largest one.
Trojan's lineup includes the DC500ML, which weighs 332 pounds for 500 AH. This is just about the same weight as 5 group 27s, for 106 AH each.
If you have a specific place you must put the battery, it will thus be the largest that will fit. Measure the space, compare dimensions against industry group sizes and manufacturer-specific sizes (like Trojan's T-series). Also consider how you will lift it into place.
I pair up group 27s because they are the largest that I can lift in and out of my battery box. A single box twice as heavy just won't work for me, I can't get a small crane into position for the job.
A US Battery 185 has about the same footprint as a Group 27, but five inches taller, for twice the capacity, but also twice the weight (120 pounds vs 59). Can you wrestle that into your battery box?
Looking at your pictures now. The compartment is sized for a Group 24, which is what's in there. It is not tall enough for 6V batteries, barely long enough for a Group 27 (13 3/4 inches long). If you can close your door on a Group 27, you can gain about 20 AH capacity (100-105 vs 80-85 for a Group 24 deep cycle).
AGMs in the same sizes often have nominally lower capacities compared to flooded cell, but if you can draw them down more on each cycle without reducing overall life, you might gain a little more capacity. Used the same as flooded cells, they often last longer (the maintenance issues) so that might be traded as more energy per cycle at the expense of some of that life.
