Notable riders who've skipped 125/250 SX

All good mentions thus far! Here's some digging on them and some extras:

*Pierre Karsmaker - The 1973 500MX champion was not only was he the first winner of the first ever SX, but also the first ever SX championship in 1974? Marked with an asterisk because SX was in it's infancy and the 125 class didn't exist until 1985. Though it's worth mentioning the 500 SX championship took place in 1974 and 1975, so would this make the 500 class the premier class and the 250 'small bikes'? Surely a moto historian could elaborate on this but I thought it was worth bringing up.

*Denny Bentley - Asterisk placed due to the 125 class not existing until 1985, but after @web_mx mentioned him I had to see this for myself. Bentley's 5th place debut at 1981 Daytona marked the top finish of his entire SX career, beating the names of eventual sx titans like Marty Smith (RIP), Mark Barnett, Mike Bell (RIP), Chuck Sun, Warren Reid, David Bailey, Broc Glover, Johnny O'Mara, Jimmy Ellis, Kent Howerton and more! He only returned to the top 10 a handful of times afterwards in SX but managed to find some solid rides outdoors (including a career best 4th at 1983 High Point), before hanging up for good in 1985. Neat!

Jeff Stanton - '6 time' went straight into the deep end to start his pro career (his first pro race was 1986 Washougal in the 500 class!) and raced 250SX in 1987. Stanton had an uneventful debut (20th) but scored his first podium (2nd) in his 6th SX start at Daytona (another coincidence, Prado scored his first podium in his 6th start). Stanton struggled throughout 1988 and didn't return to the podium until his 20th start in 1989, then won his first SX race in his 26th start (Atlanta 1989). 

Jean-Michel Bayle - After winning the 1988 125 world championship, JMB experimented with 250SX (as well as 250MX and winning on his debut!) before going on to win the 1989 250 world championship. With a rocky start at A1 (21st), JMB rebounded to a top 5 the next weekend (Seattle 1) and got his first podium (2nd) in his 5th start. By 1990 he was stateside full time, scoring his first win in his 15th SX start, finishing as the runner up in the season. JMB went on win the 1991 250 SX championship and padded his dominance that year with the 250 MX and 500 MX titles (a feat that will permanently remain undefeated due to the removal of the 500 MX class in 1994). After departing for MotoGP in 1993, JMB ended his career with 16 SX wins and one SX championship after 3 seasons, leaving a lot of unanswered questions on how much further could he have gone in the sport.

*Damon Bradshaw - The beast from the east rocketed to a 3rd place in his 250SX debut at San Diego back in 1989, but dropped back down to the 125s for the east coast championship (later winning his debut and the title) so this one gets an asterisk. After transitioning to the 250s for good, Bradshaw was victorious on his 2nd start in the premier class at the 1990 Anaheim opener (a feat eventually repeated by Ken Roczen), then backed it up with a win a week later in Houston.

Stefan Everts - Like one of his predecessors, the 1991 125 world champion opted to use 250 SX as an off season warm up going into the 1992. Everts logged back to back 9ths at Houston and Anaheim, which would remain as his best finishes and only top 10s of his 6 race SX career. Unable to verify if Everts missed qualifying for the Orlando opener, but this discrepency is a moot point as he went on to have a legendary career in the GPs.

Greg Albertyn - Another multi time world champion who was highly saught after following his 1994 world championship. The South African landed in 12th on his debut but struggled to find his footing indoors. After two lackluster seasons without reaching the podium (largely due to breaking his leg in 1996 at Houston), Albertyn struck gold at the 1997 Los Angeles opener, his first and only SX win of his career. Albertyn would find success outdoors, including winning the 1999 250MX championship, but found himself unable to replicate his moto success in stadiums, though injuries arguably robbed him of greater results.

Shayne King - Before his 1996 500MX world championship, King did one lone 250SX race; 1992 San Jose, where he finished 16th. 8 years later, King contested the 2000 250SX series and qualified for 8 main events, replicating his career best 16th one other time. He faired a bit better outdoors that year and was fast (best moto finish was a 6th at High Point), but lots of inconsistency to his results. Was he a crasher or did he have a lot of bike trouble (he did race for KTM in their early years after all)? Would love to know the lore on this. Shayne returned to New Zealand and enjoyed domestic success in the New Zealand MX championship, winning the 250MX & 500MX titles in 2005, before retiring in 2007.