Three Generation Ship novels that I liked in order of preference are:
Brute Orbits by George Zebrowski
Prisoners, political dissidents, the insane and other problem segments of society are rounded up, dumped into hollowed out asteroid ships and sent out into space.
Learning the World by Ken MacLeod
After a 400 year journey the gen ship But the Sky, My Lady! The Sky! reaches it's destination to find some surprises.
Ship of Fools by Richard Paul Russo
The starship Argonos wanders for hundreds of years looking for a new home.
All 3 are good with Brute being the best IMHO.