Ikaruga is definetly one of the best SHMUPS, but thats not to say there aren't games as good or better than Ikaruga out there. Personally i'd reccomend;
Dreamcast
Border Down (rock hard horizontal SHMUP from G-rev) -
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Gigawing 2 (awesome vertical manic shooter from Capcom)
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Last Hope (very difficult horizontal memory shooter)
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Triggerheart Exelica (vertical shooter with unique anchor gameplay mechanic, well worth checking out - also available as a downloadable title for XBLA)
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Under Defeat (amazing vertical helicopter shooter from G-rev)
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PS2
DoDonPachi Dai-Ou-Jou (ps2 sequel to Cave's manic vertical shooter series DonPachi)
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ESPgaluda (really enjoyable manic shooter from Cave)
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Ibara (rock hard manic vertical shooter from Cave)
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Mushihime-sama (another crazy vertical manic shooter from Cave, this is personally my favourite of all Cave's titles)
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Saturn
Batsugun (the originator of the manic shooter genre from Banpresto)
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Battle Garegga (amazing manic shooter from Raizing)
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Blast Wind (awesome vertical manic shooter from TechnoSoft)
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Cotton Boomerang (really enjoyable horizontal cute-em-up)
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DoDonPachi (vertical manic shooter from Cave)
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Game Paradise (bizzarre vertical shooter set in a videogame arcade)
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Layer Section (vertical space shooter from Taito)
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Radiant Silvergun (One of Treasures masterpieces, if you enjoyed Ikaruga you should love RS)
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Sengoku Ace (awesome horizontal shooter from Psikyo)
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Skull Fang (nice vertical shooter from Data East)
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Soukyugurentai (amazing vertical shooter from Raizing)
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Thunderforce V (highly reccomended horizontal shooter from TechoSoft)
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