it depends on what kind of smut you are looking for
Eiyuu Senki is more of a vanilla harem thing
Sengoku Rance has a rapist protagonist (and is part of a greater continuity, Sengoku is the 7th game in the series)
Venus Blood is tentacles and corruption all the way
if you're talking about the gameplay
Eiyuu Senki is pretty simple, recruit troops -> march to a battle -> replenish troops -> repeat
Sengoku Rance is that plus building fortifications (not very in-depth tho)
Venus Blood is more of a recruit-generic-heroes and grind them up afaik (haven't played this one)
for the versions
Eiyuu Senki vs Eiyuu Senki GOLD, the latter is more of a sequel/remake, doesn't improve on the gameplay afaik but has an entirely new storyline
Venus Blood games (Frontier, Hollow, etc) are different games entirely