I warned you it was going to be a long post. Here it comes.
I've travelled a long way with this seiyuu, and here I am. At first, he was completely unrecognisable to me, to such an extent that whenever I wasn't able to tell the seiyuu, I joked that it was probably Morikawa in question :D (And sometimes I was right indeed.) However, now, after all those years, I can tell him by a one word, so you may give me a praise. Of course, it may be all my imagination, until the next time I fail to recognize him ^^
Toshiyuki Morikawa was born 1967, Jan 26, and he lives in Kanagawa. He has some unusual hobbies that I'd rather not comment, so let's move to the important things. He is very often used by anime producers to play multiple characters in one anime, and that is possible due to his ability to speak in many different ways. (I wonder whether they pay him normally or depending on numbers xD) Many seiyuu can do it, with prominent examples of Namikawa and Sakurai, and so does Morikawa. No wonder you have problems recognizing him if he can perform in many different tones. However, before I go into anime, I'd like to mention that Morikawa voiced Obi-Wan Kenobi in Star Wars Episodes 1-3, and even though it's hard for me to imagine the character speaking with anyone else than Ewan McGregor, I admit that Morikawa probably succeeded in it ^^
I probably first heard him in "Yami no Matsue", where he portrayed Seiichirou Tatsumi, the most responsible of all shinigami, as well as very caring about Tsuzuki. Morikawa often plays responsible and caring characters, and he is probably fond of Seiichirous, for he also starred as Seiichirou Aoki in "X" tv-series.
The next time I heard him would be in Naruto, where he played two very different characters: Kimimaro, a loyal subordinate of Orochimaru and thus the enemy, and Minato, the Fourth Hokage, the good guy. As Kimimaro he is quite cold, merciless, and dead serious. As Minato he is very warm, caring, and smiling. Being as inexperienced in the seiyuu subject was I was in 2008, I would have never guessed the two of them shared the same voice.
It was somewhat the case, too, in "Bleach", where Morikawa played three characters: Isshin "Baka-Oyaji" Kurosaki, father of the main hero, Tsubaki, one of Orihime's Six Spirits, and Tousen Kaname, one of the adversaries. Here, again, we can marvel at Morikawa's skill to voice such different characters. Isshin is highly enthusiastic, very energetic, and crazy about his little daughters. Tsubaki is harsh, infuriated with Orihime's incompetence and emotionality. Tousen is quiet, always composed, loyal to a fault, and relentless in reaching his goal.
However, it was that role that blew my mind and made me admire Morikawa for ever: Katakura Kojuurou from "Sengoku Basara", the Right Eye of the Dragon, the most loyal retainer, the best friend, and an excellent strategist of Date Masamune. In duet with Kazuya Nakai, they are perfect. Never before and never again did Morikawa played a character like he played Kojuurou. In this one role he presents a whole spectre of tones and emotions: he can be calm or enraged, he can be happy or angry, he can be caring or admonishing, he can be understanding or menacing. And he is convincing in every one of them. (Because in Minato's case, for example, I sometimes found Morikawa's performance boring and unconvincing.)
Conrad Weller from "Kyou Kara Maou" is quite a similar characters, but he lacks that sternness of a samurai, and his lord, as opposed to Masamune, is still very young and thus requiring more smile and less rigidity. Conrad is a "milder" version of Kojuurou: a one being able and permitted to show his affection.
As proved with Tousen or Kimimaro, Morikawa is quite able to play not-so-positive characters, too. Alex Row from "Last Exile" isn't exactly a bad guy (well, he isn't), but he's a very dark, quiet and secretive person nonetheless. I really like Morikawa in that role, as he is unlike anything I'd heard until then. I has a similar feeling upon hearing him as Sasaki Isaburou in "Gintama", a chief commander of Mimawarigumi, the other police force apart from Shinsegumi, and an elite one. Here, Morikawa is very aloof, lordly and very impassive, almost to the extent of being insufferable. However, there's more to Sasaki than meets the ear, I believe.
Of course, he has no problems with playing jidai-drama characters (well, after what he did with Kojuurou it's really no wonder), like Matsudaira Katamori in "Hakuouki", Hijikata Toshizou in "Ryo", or Sanada Yukimura in "Brave 10"...
...or yaoi roles, like Takatsukasa Chihiro in "Kirepapa", Aizuki Keiichirou in "Winter Cicada", or Iwaki Kyousuke in "Cherished Spring" (but he was quite boring in those).
It seems to me that Morikawa's popularity is only growing, and he is going to appear in more new and successful anime. Recently, he's been given a role of Igor in continuation of "Durarara!"
My Top 3 of Morikawa would be undoubtedly Kojuurou, then probably Conrad, but I have no clue as for the third place, for there are so many roles that I truly admire. So maybe I should wait for something new that would make a great impression on me ^^ On the other hand, in regard to increasing popularity of Morikawa, I fear whether he's not going to become another Shin'ichiro Miki, playing in every other anime you watch. I really wouldn't like to get bored with Morikawa... but now that I've cleared all his possible way of playing I wonder whether he can still surprise me with anything.