I won't even attempt to go in depth (there is an 
art difference example below, though), but this 
just lists all the series.

OVA (1st series [6 eps], 2 specials, 2nd series 
[6.5 eps], the upcoming 3rd series, Movie #2 [the 
addition of Kiyone here is odd])

1st TV (26 eps, Movie #1, Movie #3)

2nd TV (26 eps, 1 special)

Pretty Sammy OVA (3 eps) - spin-off series

Pretty Sammy TV (26 eps) - spin-off series

There's going to be yet another spin-off series 
based only on the Galaxy Police.

Below, you can just compare 2 Ryokos (in 
battle/space pirate garb) that I have from the 
1st OVA and 2nd TV series (shin tenchi muyo).  Of 
course, you can browse through Division's site 
for a more comprehensive view of Ryoko from all 
the series/movies (although some are placed in 
the wrong category, heh).