QR or Guard?

If you want to run him as a super offensive Enemy Phase unit that can proc a massive Igins bomb every round of combat, then Quick Riposte and Close Def are the best skill combinations. However, this build has no sustain whatsoever and is mostly only good for 1 or 2 rounds of combat.

If you want your Lukas to last multiple engagements, then I think that Guard + Quick Riposte might be better. He is almost always going to get doubled, meaning that the enemy could get up to 3 Special charge--up to 5 if it is running Heavy Blade or Flashing Blade--off of him, which means he could eat a powerful Special on the enemy's second hit without Guard. The >80% Guard threshold shouldn't be that big of a problem since Lukas' Def is massive, allowing him to take single digits from plenty of non-Green physical units, but having a way to recover HP (by Healer or pseudo-Healer) is going to improve this build (and the other one) immensely.

The best enemy phase set for Lukas's talents would be Aether+Steady Breath+Guard+QR Seal, if you're willing to make it (big 'if' to sac an Ike, I know). Lukas can't reliably defend against middle-of-the-road speed Dragons with just 1 Close Defense skill (though he can defend against weak blue ones and red ones if they're built to proc aether upon being attacked if he has a +Res refine on any weapon and he baits them), though if you gave him Berkut's Lance, he might be able to handle them.

If a unit isn't prepared to handle at least some manner of dragons then Close Def is actually rarely the purely optimal A slot for it, unless that unit somehow has special acceleration from another skill. (Even Steady Stance[*edit*] would be better, as it'd trigger on bows and daggers.)