Have Big Game - like it and play it alot. Hard pin.
Had Vector - good game, miss it...but the shots are limited.
Game settings for guardian targets are key - as that really changes the difficulty.
Circus (passed on chance to own a good one) - Way, way too easy, and roto targets blow.
Had Elektra - good game, and again--game settings really change it up.
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Big Game would be my choice...but I would add Elektra too if $ allows.