i would say skill, but i have got to say a mixture of both for a couple of reasons. i'll list them as why:
Skill
-you should be able to beat the game with practice, and the better player should be able to beat the weaker player. if the game has no element of skill it will just be frustrating.
-there comes a great deal of satisfaction knowing you beat something hard, because you are that fucking good. if its down to luck entirely, you won't be as satisfied.
Luck
-finding something knew, discovering a new tactic from random button pressing, it may become skill in time, but first time it's normally chance, or luck. then afterwards you're like "dude, what did i just do?!" and so you go on to work it out, developing that lucky streak into a skill.
-in an encounter it is nice to have certain things like crits. i mean, it really will keep you on edge if you think its a walk in the park, then suddenly BAM! half your health has gone from 1 random mob! i mean, it can be a dick move sometimes, and something will just 1-shot you (lucky headshots from CODspraying are always annoying) but if it makes things a lot more intense, i don't see why random 'luck' events should be removed.
so, arguments on both sides here. overall i think skill based gamesa re more rewarding, but luck elements shouldn't be removed entirely.