the most uncommon skill cape is probably the "quest skill point cape" (need name confirmation) as one can only obtain it when one has sucessfully completed all quests and that new quests are always being added, making it difficult to obtain
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Whoever posted the above information is wrong. The quest point skill cape is not even close to the rarest. When a new quest is added, the quest point skill cape stays in the inventory/bank of the person, so they will still have the cape despite not completing the newest quest. The rarest skillcape would be runecrafting.
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Firemaking, though it is very time consuming, it is officially the most cheapest skill cape there is available, only to Quest Cape ( Which isn't even classed as a skill cape ).
all reqular capes have the same defence stats exept ( i think skill capes ) so really any cape is good just chose the one that you like the most
If you go on runescape highscores, you can see that lots of people have 99 skill level in everything. Every single one of these players will have all the skill capes
Walking is not a skill at all. However, your Agility level effects how long you can run, walking however is not and most likely will never be incorporated as a skill in RuneScape.
You can find this out quickly in the game itself: click on the skills icon, and click the "Defense" skill - the relevant skill for wearing most armour. In the case of adamant, the answer (for the new RuneScape) is that you need Defense level 40.