The only two goals I have seen so far are that you want chickens for eggs and you plan to only keep one rooster. You will re-home the extras and have lots of room though I'm not sure how you will use that room. You are in Iceland. I understand that was considered a temperate climate but that was by people used to Scandinavian winters. So I'm not sure how much of the year they will have good access to the outside so they can forage and play outside. Probably more than I'd expect.
Roosters don't lay eggs so I'm still not sure why you want a rooster. That means I don't have any criteria to help you make that decision. The only reason you need a rooster is if you want fertile eggs, anything else is personal preference. Nothing wrong with personal preference, that can be really strong. But it is a choice, not a need.
If you don't want him for fertile eggs and one doesn't really stand out for eye candy or whatever reason you want him, go by behaviors.
If you want him for fertile eggs select the one you would want his offspring to be like. Color and pattern can come into that depending on what colors/patterns your Sussex are but when you crossbreed you can get different things than you'd expect. I'm thinking more on behaviors. Since you are not eating your excess cockerels I'll assume size isn't important. If you keep a Legbar male all his offspring should lay colored eggs, if you keep a Sussex only the chicks from Legbar hens will lay colored eggs..
Your chicks are 12 weeks old. You may already see some differences in behaviors between your cockerels and pullets but fasten your seat belt, you haven't seen anything yet. When the hormones hit the cockerels their behavior will change. Each chicken, cockerel and pullet, have their own personality so you can get quite a bit of differences in individual behaviors, including as they go through puberty. Also, puberty hits some cockerels pretty early, maybe at 12 weeks, but it may not hit others until after 5 months so some behaviors can repeat as different ones hit puberty.
There is another complication. The dominant chicken tends to suppress the behaviors of the non-dominant ones. When you remove a dominant one the others' behaviors can change since the top one is no longer keeping them in check. Sometimes the presence of rivals can stir the dominant one into more severe behaviors.
It can get really complicated when you have mature chickens in with them (maybe next year for you) but with yours this year it will be the cockerels. When the hormones hit the cockerels will want to dominate and become flock master. They do that by fighting and intimidation, but also by mating. When they mate the one on bottom is accepting the dominance of the one on top, either willingly or by force. Pullets are not likely to be willing until they mature enough to start laying so it will almost certainly be by force. It might be one dominant cockerel, it will most likely be a few. You may see fights between cockerels, you may see more running and chasing between the cockerels. Almost certainly you will see cockerels chasing and forcing pullets to mate. It doesn't have anything to do with sex or fertilizing eggs at that stage, it''s about dominance. To me, to a certain extent, this is normal behaviors. As long as no one is being injured I stay out of it. But it can be really hard to watch, especially if you haven't been through it before. I typically hatch and raise about 45 chicks a year, cockerels and pullets together. They have a lot of room, which I think helps. About once every three or four years it gets rowdy enough for me to separate out some cockerels to their own grow-out pen until they reach butcher age. I do watch it and take action based on what I see. Typically when the cockerels mature enough they stop this hormone driven behavior and mature into real roosters. The pullets mature enough to do their part. Once they all mature it typically turns into a very peaceful mature flock, but watching them get there is often not for the faint at heart.
Then there is the human aggression part. It's not that easy to write about. A young cockerel may test his limits with you soon after the hormones hit. We all handle this differently. For some that is an immediate disqualification, I don't blame them for that. I allow a very small amount of posturing but walk toward him until he backs down. They are typically cowards and try to ambush you. If you calmly and slowly walk toward him and give him time to back down he usually does. If he doesn't learn after a couple of times dong this or if he actually attacks me he is gone. Some others are more patient with this behavior than me. I think your attitude has a big effect on this too. If yo are afraid of him he will know and be more aggressive. As I said, other people handle this differently.
The way I decide which one to keep is to first know my goals. My goals are to raise them to eat so size and early maturity are important. I also play with genetics, so certain things there are important to me. I will use them for breeding. So I remove any with obvious flaws or physical problems, plus any that don't suit my goals. I also watch behaviors. I consider certain fighting or mating behaviors normal and natural, but if I see one that is exceptionally brutal he is gone. When cockerels fight it usually doesn't take that long for the loser to decide he's better off running away. I expect the winner to do a little chasing to reinforce his victory but if one is relentless in chasing he's overdoing it. A cockerel will force a pullet but if she is injured, he did too much. It's a matter of degree. It's usually not that hard for me to reduce my 15 to 25 cockerels down to two or three, then it gets hard. But by that time neither of them is that bad of a choice.
One of the problems with this is that cockerels are typically bolder and braver than the pullets, they have more personality. Also, males typically feather out prettier than females. These are usually the ones you fall in love with. Sometimes you have to be strong to let them go.
So my basic suggestion is to determine your criteria based on your goals (not mine) and remove any that disqualify themselves from the flock. You can get rid of them immediately or build a separate pen where you can house them either by themselves or with other cockerels until yo are ready to get rid of them. It's not usually that easy a choice and I'll admit I sometimes make mistakes. Behaviors can change as they mature or when you remove another.
Good luck!
. Consider culling the males that begin to look at you like a threat or actually show you aggression (unless they were truly surprised by an unexpected event), aggressiveness to the girls where they are keeping them from eating, patrolling the coop entrance and then jumping on the girls when they come near, etc. These types of behaviors may begin to surface by 14 weeks -20 weeks typically. 
