Best Game From Ultra Bundle?

If I had to give my honest opinion, I'd say Droid Assault was the best one. You basically capture enemy droids, to use as your own. You can use an entire army, or go solo. If you have enough points, see a nice strong enemy troop? Capture it, use it for yourself!

Revenge of the Titans is a tower defence game.
Titan attacks is Space Invaders with character progression.
Ultratron is an arena shooter. Think something like Smash TV.
Droid Assault is perhaps the most original of the lot. Deiru described it pretty well.

Out of the four I'd say Revenge of the Titans is probably the most fleshed out of the four. Has a full lengthy campaign with tech research etc. But my personal favourite is Ultratron. I guess it will depend on which genre is most appealing to you.

Here's my brief take on the games:

• Revenge of the Titans: RTS/Tower defense blend. Mine resources to get money to build towers, bots, and other defenses to defend your base from waves of aliens. Technologies unlock between levels. Gets very difficult at later levels.
• Revenge of the Titans: Sandbox Mode: Allows playing user-made levels
• Titan Attacks!: Space invaders with upgrade abilities between short levels. Simple arcade/casual game.
• Ultratron: Top-down shooter that you spend points on upgrades. (I've only played the demo of this one)
• Droid Assault: Another top-down shooter where you assimilate enemies to join your team to destroy other enemies.

I like revenge of the titans. (have it drm free) If you like tower defense, you'll like this. and the sandbox dlc is worth it.

Have them all, I'd say my favorite is Droid Assault, but I bet I've spent more time on Titan Attacks!

Revenge of the Titans mostly just kicks my ass.

I've only played Revenge of the Titans so far, but I can say I really enjoyed what I have played, I played it for the winter achievement in 2011, and going into that blind was both frustrating and rewarding when I finally beat it, played it some after that and seems to hold up.