I would say any of the last stand games by Con Artist. And the Epic War series is a great one as well. It really depends on the types of flash games you like. I tend to lean towards the strategy games myself. A good over all game that others have mentioned is Elephant Quest.
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well if you like zombie rpgs i suggest you play The Last Stand-Union City its really fun but if your like me you will jump when the horde comes but you will see for yourself
I'm usually not a fan of zombie games but Boxhead The Zombie Wars really cracked me up. Build a base while blowing up everything with minigun and rocket launcher, when all seems to be lost, call for nuke, pick up the stuffs and keep the party going.
Also enjoyed King of Drift, it is too hard for most people but if you manage it...
For action RPG I like Barbarian Onslaught The Secret of Steel most. It is simple, character doesn't level up, but it feels that you are actually the one who leveled up as the stages get harder and harder. It also has epic BGM.