That's something unique about Pokemon: many different artists can depict the same Pokemon in their own style. Kagemaru Himeno, Mitsuhiro Arita and Kouki Saitou are definitely some of the best artists. Here are some some of my favourites which haven't been mentioned in the posts above:
Hajime Kusajima:
This artist is very versatile - he can go from bold illustrations to soft watercolour illustrations. My favourites: SS Sableye, Emerald Exploud line, UF Thyplosion line, DS Dratini and Dragonair, DX Plusle and Minun. There are a lot more, but I won't list them here
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Midori Harada:
His illustrations have very muted colours - but don't always turn out well. My favourites: HL Banette, SS Cradily
Naoyo Kimura:
His ilustrations always have nice soft colours. They should get him to make more illustrations - and put Miki Tanaka and Yukiko Baba out of their jobs. My favourites: TRR Ratatta, SS Absol
Sumiyoshi Kizuki:
His illustrations lack shading, but have a unique style: My favourites: HL Castform line (Snowcloud Castform was the best), TRR Azumarill, TRR Togetic
Kyoko Umemoto:
Like Kizuki, his illustrations also lack shading. A poorer counterpart to Kizuki, Umemotos works can range from good (my favourite: SS Illumise, FRLG Wigglytuff, HL Electrode) to just plain ugly (DX Lunatone and Solrock)
Aya Kusube:
For some reason, I don't like his newer style beginning from the TRR set onwards. His crosshatching technique is very nice. My favourites: FRLG Shellder, Cloyster and Cubone, HL Chinchou, DX Sableye
Emi Miwa:
His illustrations were only featured in one set: Team Rocket Returns, but I think he's responsible for the FRLG booster pack art as well. My favourites: Dark Ampharos, Dark Magneton, Dark Dragonair.
Woah, that's one long post...