Thread got me thinkin... I would go with Uncharted 2. It is the game that made me realize the potential of storytelling in video games. It might seem derivative by today's standard, but you gotta realize that Naughty Dog are the guys who really popularized that formula.

Yeah, Uncharted 2 probably
or Batman: Arkham City

First thought is Halo 2

came itt to say Uncharted 2

Mass Effect 2

pokemon gen 2. black ops 2. sonic adventure 2 (battle).

God of War 2
Call of Duty 2
Halo 2

Half-Life 2

Mafia 2

MW2

This and portal 2

Surprised no one has said red dead 2

I actually enjoyed the first one a lot more

skyrim, resident evil 4, pokemon gold and silver

KH2

Mario 64
Metal Gear Solid

AC2 was such a big leap from the first game

The Witcher 3

Mass Effect 2

The first ones that come to mind are

Mass Effect 2
Portal 2
Super Smash Bros. Melee
Halo 3
Kingdom Hearts 2

Picking just one of these is tough for me lol

Title said sequel not trequel