I have collected a list of challenging airfields all over the world (ICAO Airfield, Country). Those are mostly for airliners (not for bush flying). I haven't been able to find FSX airfields for all of those. Propably there are much more of these airfields in the real world.
BGBW Narsarsuak, Greenland
BIIS Isafjurdur, Island
EGBR Barra International, Scotland
EGLC London City, GB
EKVG Färö, Denmark
ENHV Honningsvåg, Norway
ENSB Svalbard, Norway
FXME Matakane, Lesotho
KCLR Cliff Hatfield, California USA
KDCA Ronald Reagan, Washington USA
KLGA La Guardia, NY, USA
LFLJ Courschevel, France
LIPB Boizano, Italia
LLMZ Mezada, Israel
LOWI Insbruck, Austria
LOWZ Zel am Zee, Austria
LPMA Maderia, Portugal
LSZA Lugano, Swizerland
MHTG Toncontin Teguicigalpa, Honduras
NZMF Milford Sound, NZ
NZQN Queenstown, NZ
OPGT Gilgit, Pakistan
PADK Adak, Alaska, USA
PADU Unalaska, Alaska, USA
RCLY Lan Yu, Taiwan
RPVE Caticlan, Godovedo P Ramos, Philippines
SBRJ Santos Duomont, Rio de Janeiro, Brasil
SBSP Gongonhans, Sao Paulo, Brasil
SEQU Mariscal Sucre, Equador
SESM Shell-Mera, Equador
SLCN Charana, Bolivia
SLSU Sucre, Bolivia
SPNP Ventilla, Peru
SPRF San Rafael, Peru
SPVI Vicco, Peru
SPZO Cuzco, Peru
SVMD A Carnevali, Venezuela
TDCF Canefield, Dominica
TFFJ St Bart Gustav III, Saint Bartholomey
TNCM Princess Juliana, St Martin
TNCS Juancho E. Yrausquin, Saba Nethrlands Antilles
TVSM Mustique, St Vincent & Grenadines
UIKB Bodaybo, Russia
VHHX Kaitak, Hong Kong
VI66 ****he, India
VISM Shimla, India
WMBT Pulau Tioman, Malaysia
WMBT Tioman Island, Malaysia
VNLK Lukla, Nepal
VNSB Syanboche, Nepal
VQPR Paro, Bhutan
ZUNZ Lin Zhi, China
Pekka