what are your favorites? and by 'pure survival', I mean more Minigore style versus a real deep game like Bug Heroes...
Solomon's Boneyard was good when it came out but it might feel a bit outdated now compared to some of the newer ones. If you don't mind that, then it'd be worth checking out.
Max Adventure is really for "pure survival" by sending the attacking aliens back to save the planet. It has endless adventure, huge levels to explore with hidden areas and varied mission types, all of which make the title a deep gameplay.
iDracula--A Classic Infinity Field--Just good fun Geo Wars: Light Fighter--Neat weapon system Geometry Wars--Inspiring for all ios devices; granddaddy of Dual-Stick shooters (when it was on consoles) and of course Dead Ops Arcade (I know its not out for iOS, but it should be lol. Its better suited for iOS than it is for consoles, which it does well nonetheless)
For short playing sessions, I like Minigore (on insane mode) or PewPew (for the different play modes). Each session for me lasts just a few minutes. While I really like Max Adventure, I somehow haven't come back to playing this after beating the stories and unlocking the items I want. When I have longer playing sessions (over 15-20 minutes), the ones I keep going back to from time to time is Gun Bros (if it weren't for all the upgrades, I'd probably have stopped playing long time ago) and definitly both Soloman's Keep & Boneyard (longer & "deeper" experience with all the different level up upgrades). I want to get Age of Zombies (liked the lite version), but I am currently a bit burned out by dual-stick shooters. I'll wait a little bit before getting this.