Cloudrider + Clockmaker + Teleport = ZOMGSUPERSPEED
*Votes those 3 as most fun*
Lol, move 7 hexes per turn :]
Let's keep looking for skill combos:
Summon Creatures + Duplication -> You can create 12 cards for each duel, so you can make a legal deck even if you have only 18 cards in your vault.
Chosen One + Stall -> Discard 12 less cards after a loss.
Chosen One + Stall + Low Point of Gravity + Champion of Gravity -> Discard 15 less cards after a loss.
Energy Barrier + Dust Cloud -> Use 2 relics and the town has a 40 hp force field.
Calm Before the Storm + Phase Shifter -> You can safely gather counters and return to finish the battle later.
Calm Before the Storm + Stealth -> Also helps to guarantee safety when you want to gather some counters.
Treasure Hunting + Research -> Use 5 AP to gain a Tier 1 skill for 6 turns.
Backstabbing II + Uncontrollable Rage + Critical Strike II + Heretic + a whole bunch of other skills -> A defeated 2-0 opponent has to discard 22 cards, a relic (and a lot of other nasty things happen)
A number of skills I'm not going to call by name -> You're gaining XP like mad.
Of course some of these combos would require such a high level (and taking other, weaker skills) that they are totally not worth aiming for. Still, it's fun to find the possibilities.
By the way, wouldn't it make more sense to change the skills that give you once 6 cards (ie. Doppelganger, Tremors, Stellar Radiation...), to work similar to Summon Creature?
For example, instead of
"After learning this skill, gain 6 copies of the Supernova card."
it could be
"During a duel, you can use up to 6 Supernova cards for free. They disappear after the battle."
This way it cannot be exploited through selling 6 upgraded cards and buying a dozen of unupgraded cards to gain quickly a big vault. On the other hand it would mean that if you lost those cards for some reason, the skill wouldn't be wasted. IMHO skills should reflect skills, abilities, spells the Elemental can cast etc. rather than reflecting the cards he has in his vault.