I really liked Isotope. It has simple graphics yet good effects, and lots of replayability since there are RPG elements like stats and money.
If you want a space shooter,meteor blitz is the clear chice! It has a story mode, survival and seamless gameplay. If you want on foot survival shooter, get bloodnguns. It is more or less like an awesomer version of idracula as It has pickups, multiple guns and unbelievably intense gameplay as well as 6 game modes. Another good shooter that not many people know about is archrobo. The graphics aren't great but the music and gameplay is really fun. For free, you could get pew pew or paradise
Annihilation Undead is surprisingly fun... Nice amount of weapons. I've only played the lite version but I'm considering getting the full thing after playing it. BloodNGuns is also alot of fun. It feels just like Endless Zombie Rampage, which is one of my favorite flash games. Good amount of variety in the game modes and weapons. iDracula. No explanation needed. Project Phoenix. Just so much going on it's awesome. You need it. All really good dual stick shooters.