I'm just writing this list based on who's voice I love. Despite listening to a lot of drama CDs and being a seiyuu fan in that sense, I seldom actually look for info on seiyuu or watch events. In fact I actively try to avoid seeing pictures of them because if I know how they look that starts to clash with the character image in my mind. So I can't really say much about their personality and things like that. I know some people are also fan of certain seiyuu (or actors for that matter) because of who they are, I just love them for their voice or acting skills.
(I'm really sorry if this is all very boring and stuff, but I promised I would write this so I wanted to make a proper post out of it)
1. Sakurai Takahiro
Sakurai has been at the top of my list for years now, with the one and only argument that I simply love his voice. The cause for my incredible Sakurai bias is one series and one series only: Are you Alice?. Sakurai = Alice (and vice versa) in my head. My love of his voice has reached the point where I still want to see Twilight in Japanese just because he voiced Edward. (I'm... not much of a Twilight fan in case you didn't know).
If we are not talking about drama CD my favourite Sakurai roles are probably Asbel Lhant (Tales of Graces), Clause Faustus (Kuroshitsuji), and the Medicine Seller (Mononoke), though I tend to go into fangirl mode as soon as I hear his voice.
Favourite drama CDs: everything Are you Alice (but especially the tokuten or anything where Alice acts out of character), Anata ga Ofuro de Noboseru CD Vol.1 Kusatsu, Fujoshi no Totsugikata / Fujoshi no Mikata, Fushihi Koubou Vol.3.
2. Hirakawa Daisuke
Second one without a valid argument. Hirakawa-san has a pretty interesting voice range, and I love both ends of it. When he plays hetare or clumsy characters, they are just so adorable that I cannot bring myself to dislike them no matter how clumsy they are. I prefer him using the lower ends of his range though, but sadly I don't know many examples. At either rate, I love it when he voices (slightly) dangerous characters.
I think I first really noticed him as Ace from Heart no Kuni no Alice, but what really sparked my fandom was Yandere Heavon vol.2. His yandere performance is -if you ask me- one of the best in the series, and the reason why I secretly keep hoping that almost every character he now voices has a hidden side... Going through his list of anime roles, I think I have literally seen none of the anime in which he voices a major role. So my love is solely based on what I mentioned above, haha.
Speaking of wanting to see Japanese dubs of movies; Hirakawa-san is the voice over for Orlando Bloom, and watching the Lord of the Rings or Pirates of the Caribbean in Japanese is also still on my to do list.
While I said that I only like seiyuu's for their voice and know little about their personality, I do love how polite and gentle Hirakawa-san always sounds in cast talks and interviews.
Favourite drama CDs: Shuukan Soine Vol.1, Yandere Heaven Vol.2, Hougen Renai Vol.3, Anata ga Ofuro de Noboseru Vol.3 Misasa, Double Score ~Narcissus~, and his story in Kannou Mukashi Banashi Vol.6.
3. Okiayu Ryoutarou
I think that Okiayu-san was the first time I actually became fan of a seiyuu (before that I would like certain characters, but not specifically their seiyuu). This bias developed out of a character crush: Tezuka Kunimitsu from Tennis no Oujisama. Okiayu-san is one of the few seiyuu who's singing voice I also like, and I have several of his singles and albums (most of them are from Tennis no Oujisama xD). After I stopped watching the show the crush slowly died out though, but at some point I re-discovered him. By this time my Sakurai bias was already formed, so he is no longer at the top of my list anymore, but still a long time favourite.
When we're not talking about drama CD's, my favourite Okiayu characters are Tezuka Kunimitsu (obviously), Dark (D.N.Angel), Gaius (Tales of Xillia. So I am really really really excited that he becomes a playable character in Xillia 2).
Favourite drama CDs: Shimekiri CD, O.To.Na Gentei Otogame CD Vol.2, Oujisama (warai) series. While not technically a drama CD, the drama part from Sasayaka na Kono Negai is absolutely brilliant.
4. Koyasu Takehito
Probably the only seiyuu who's voice I can recognise with 100% accuracy even if he speaks only one line. (although there has been one incident where I failed to recognise him). Despite not being my favourite seiyuu ever, I have a habit of calling him "Koyasu-sama".
Despite the fact that he is often cast for characters with a cold and distant personality I love him more when he does sarcastic or comical roles. Especially if you can combine those two.
The first time I fell in love with his voice is probably when I was paying Tokimemo, where he voices your teacher, Himoru Reichi. Despite originally going after another character his voice won me over and I started going after Himuro halway through the game. When on another character his route it always hurt like hell when I had to reject his invitations (T-T).
I originally played Tales of the Abyss in English, so I first discovered that he voiced Jade when I watched the anime, but that role is basically my ideal role for Koyasu-sama. In Hakushaku no Yousei his role is far more comical, and I spent most of Persona -Trinity Soul- fangirling over Ryou because he was voiced by Koyasu-sama.
Favourite non-drama CD roles: Himoru Reiichi (Tokimemo Girls side), Jade (Tales of the Abyss), Julius (Heart no Kuni no Alice), Kelpie (Hakushaku to Yousei), Kanzato Ryou (Persona -Trinity Soul). His voice completely mismatches Aokiji (One Piece) his image in my mind, so I actually dislike the character because of that.
Favourite drama CDs: Are you Alice - Alice no Tea Party Vol.3 (I cannot express my love for this CD in words), Heartful Kareshi series (he voices a dove). Unfortunately he doesn't appear in that many drama CDs, but I am definitely going to track down more of them.
Fukuyama Jun and Takemoto Eiji. While I don't particularly like (or dislike) their voices, I've heard these two voice so many different types of characters that I just can't help but admire their skills.
For Fukuyama Jun it was the difference between Shimizu Keiichi and Lelouch Laperouge that first made me aware of this. Shimizu was the first character that I associated with Fukuyama, which is why that image is stuck in my head. After doing some research I discovered that I heard him in countless roles, without even recognising him. This video shows a some of those, although it is a lot easier to recognise when they are all put together. Lately I've been getting better at recognising his voice, although I often only recognise it but fail to pinpoint who it is.
As for Takemoto Eiji, just listening to the Oujisama (warai) series gives you an impression of all the different characters he can voice (he voices almost every side character in the series, this has also become somewhat of a running gag in the series).
After listing my favourite, I suppose it is only fair that I list my least favourite ones as well, although there are only two of them, and I think that they are quite obvious from various reviews that I have done (^-^;;)
Ishida Akira in situation CDs / otome games. I don't know why, but I cannot bring myself to like Ishida-san when he voices a boyfriend role. That being said, he can voice very convincing shota's, and I have less problems whenever he voices a normal role in any anime or game. It is just that I don't like boyfriend!Ishida...
Yoshino Hiroyuki whenever he uses that gyaruo/bratty voice, which is most of the time. He actually has a rather nice voice if he talks in a more serious tone, but unfortunately he is usually type cast for the former. My disappointment when I discovered that he voiced Debito in Arcana Famiglia was unending (although it is actually one of the more reasonable examples).