Snivy,Servine,Throh and Seismitoad are my favorites from Generation 5.Snivy,who is arguably the most popular of the three for what I've seen,is more for the good memories of him being my first Pokemon ever,and his final form doesn't disappoint too,with some good accuracy,speed and attacks.
I randomly caught Throh because he looked awesome when I saw him,and his defenses are quite something to be recognized with,when not fighting a majorly strong Physic or Flying type.His attacks are quite powerful as well.
Seismitoad is more of the underappreciated one from my team in-game,since most people judge him by his humongous and weird appearance.Other than that though,his strengh and attacks can be quite handy and strong,and even use some unexpected moves for defense.