Hard to pick a favorite, because it depends on the level it is played at, combinations you have for the respective splinter, and so on.
So, I'm gonna pick a little differently. The only common summoner: Delwyn Dragonscale.
A dragon summoner with +1 magic. Unfortunately it has 5 mana, and there are much more powerful summoners with 5 mana. But the combination of +1 magic + dragon splinter may be winning in some cases. I wouldn't max it though, unless I'd have tons of money and price at that level doesn't matter for satisfying some small pleasures.
This is one of my favourite cards and one of my two maxed out summoners. I bought it when it was still affordable :-). It's very powerful in high mana games. I love to put all my dragon and water magical cards together in such games.
Nice! Lots of dragons with magic attack.
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