Excluding wrecks, unfinished projects, and stuff that I ended up with in horse trades & never rode. I reckon that excludes at least half the bikes I've owned... Hel, it excludes about half the bikes in the shed now!
From 1988 to 1997:
0. 1984 ATC125ME (1984 to Xmas day 2001 [RIP]).
1. 1977 XL175 (1988 to 1991)
2. 1974 DT250.
3. 1974 DT250 (1991 to 2013).
4. 1978 DT125/175 (1991 to 2003).
5. 1972? Hodaka Wombat (1998 to xmas day 2001 [RIP]).
6. 1988 KTM125GS (1997 to 1998)
7. 1994 TM125E (NYE 1997 to current).
From there, chronological order gets too difficult...
Beta.
2013 300RR (Jan 2013 to current).
GasGas.
2003 200EC (2011 to 2013).
Honda.
1971 SL100.
1984 CR250RE (2013 to current).
1993 CR125RP (2011 to current).
Husqvarna.
1985 WR400 (2006ish).
1993 CR125 (2011 to current).
1999 WR360 (2006 to 2007ish).
2002 WR360 (2013).
2011 XC150 (June 2011 to current).
Kawasaki.
1982 KLX250 (2011?)
1982 KDX175 (2010?)
1984 KDX200B (2007 to 2013)
1986 KX200 (KX125/KDX200 hybrid. 2010 to 2012)
1987 KDX200D (2011).
1988 KX125F (2010 to current).
1988 KX125F (2011/12).
1988 KX250F (2012 to 2013).
1989 KX125G (2009?)
1990 KDX200E (2012).
1992 KX125J (2010 to current).
1994 KX125K (2009?).
1997 KDX200H/J (2010 to 2011).
1998 KDX200H (2012).
2008 KX125 (2014 to current).
KTM.
1984 125GS (2011 to 2013).
1989 250GS (2005 to 2007?).
1989 125GS (2008 to 2010?).
Suzuki.
1972(?) TS90 (2007 to 2008?)
1999 RM125 (2009)
TM.
2004 125EN (2010 to 2012).
2008 250EN (2011 to 2011).
Yamaha.
1969 AT1
1969 CT1.
1970 RT1.
1971 DT1.
1973 MX250 (2010 to 2012).
1974 DT125A.
1974 YZ125A.
1975 YZ125C.
1975 TY125B.
1976 YZ125X (still have).
1977 YZ125D (still have, for sale).
1977 YZ125D.
1977 IT175D bitsa.
1977 YZ250D.
1978 YZ125E.
1979 IT175F.
1980 YZ250G.
1982 IT250J.
1984 YZ125L.
1988 DT175.
1994 YZ250 (still have).
1999 YZ200 (WR200/YZ125 hybrid, 2012 to current).
2003 TTR250 (2012 to current).
2006 YZ125 (2011 to late 2012).
I'm sure there's more - I'll add them as I remember.