An impressive list, Akashic. It's nice to see other people who bother to spend time on their forum posts.
Let's see... For me there are two kinds of awesome seiyuu: those who do one awesome voice and those with impressive variety of voices. I'll start with the one voice people:
Wakamoto Norio"When God speaks Japanese, He uses Norio Wakamoto's voice." - quigonkenny in the MangaFox forums
The King of male voice actors. There's no competition. Hellsing is where I first heard him, but his performances have impressed me in numerous anime: Demonbane, Princess Lover, Shounen Onmyouji, Tayutama etc.
Nakata JoujiAnother one familiar to me since my first anime, Hellsing. He's awesome also in Daimaou, Escaflowne, Nurarihyon no Mago etc.
Okiayu RyoutaroAnother one with a cool deep voice. D.N.Angel is where I first noticed him, and later I've liked him in Bleach, Flame of Recca, FMA, Prince of Tennis etc.
Ono DaisukeI know he's more famous for his role in Haruhi, but to me he'll always be the guy who voiced Kazuma in Kaze no Stigma so perfectly. I like him wherever he is, but other really good roles were in Umineko, Kuroshitsuji and Yakumo. Also, not a bad singer.
Inoue KikukoShe has the most pleasant, motherly voice. Ah! My Goddess, Nadia, Onegai Teacher, Ranma ½ and Umineko saw some of her best roles.
Arai SatomiShe's freaking hilarious. She's most well known for her role in the To Arus, but for me her crowning role is as the narrator of Ookami-san to Shichinin no Nakama-tachi.
Itou KeitarouI like his voice. It's as simple as that. He's good especially in Bleach and Demonbane.
Ishida AkiraA fantastic villain voice, for example in NuraMago. Also pretty good in Bleach, Shounen Onmyouji, Naruto and D.N.Angel.
Sugita TomokazuA fairly ordinary voice, but perfect for sarcasm and some other comedic uses. I especially like his performances in Haruhi, Umineko and Shuffle.
Shimizu AiLove her voice. Greatest roles in Onegai Twins and KoreZombie.
Tachiki FumihikoHe's one of those voice actors with a very distinct voice. He's also one of those seiyuu you hear too rarely and usually in small roles. I know him from Nogizaka, Oreimo, MoonPhase, Bleach etc.
And then the ones with variety:
Oohara SayakaPerhaps my favourite female seiyuu. She's fucking awesome in Umineko, adorable in Toradora and great in Fairy Tail, Asura Cryin', xxxHolic etc.
Kugimiya RiePeople often mistake her as one of the single-voice actresses, but actually, she can do a lot more than her signature tsundere voice. See Fairy Tail, FMA and Umineko for examples.
Park RomiShe does a very impressive young man's voice, for example in FMA, Bleach and Detective Loki. However, she does an equally impressive woman's voice, for example in Naruto and Kuroshitsuji.
Fukuyama JunHe's the one they always get to play the young male character who has two different voices, such as in NuraMago, Code Geass and Rental Magica. Often he also plays good one-voiced roles, such as in Kuroshitsuji, xxxHolic, Spice&Wolf and Ao no Exorcist.
Horie YuiQuite often her voice is very ordinary and unrecognizable from among a million other voice actresses, but sometimes she shines. In my opinion her role as Maria in Umineko was the performance of her life. She's also a good singer.
Shinichirou MikiHe always plays such fun characters, for example in Bleach, Hatenkou Yuugi and Full Metal Panic. He's also an outstanding seiyuu and a pretty good singer.
Ookawa TooruCurrently great in Sacred Seven. Also good in FMA, Tears to Tiara and Trinity Blood.
Nakahara MaiAnother one with a fairly ordinary voice, but some really good roles, such as in Amaenaideyo, Zakuro, Higurashi and Katanagatari. Also a great singer, I love her character songs in Zakuro.
There are a bunch of others I like, Fujimura Ayumi, Kitamura Eri, Itou Kanae, Irino Miyu, Satou Rina, Kamiya Hiroshi, Abe Atsushi, Noto Mamiko, Hanazawa Kana, Tamura Yukari, Touchi Hiroki, Nakai Kazuya, Nonaka Ai, Toyosaki Aki etc. But, this has already taken hours, so... only the very best got a proper introduction.