3d GTA trilogy(GTA 3,vice city and san andreas):-
These 3 games defines the ps2 era for me they popularized sandbox genre and no matter what anyone says not even watch dogs can top what san andreas did in terms of "Grandness".
Hitman Trilogy(Hitman 2,contracts and blood money)
IF you are a PC gamer from the early 2000's you must fondly remember hitman 2 and contracts! and blood money is like a polished&refined collage of those two games.
Metal Slug(1,2 and 3) :-
Simply put metal slug is the best side scrolling run'n'gun games i've ever freaking played! contra seems visually dated and stiff in comparison to this.
God of war trilogy :-
This game series is easily one of the best action adventure/hack'n'slash game series if there's anything that makes this trilogy seem any less perfect its the amount of unwanted prequels it keeps getting.
Silent hill Trilogy:-
Silent hill trilogy makes up for all the shortcomings of resident evil in ps2 era but unfortunately after 3 brilliant games the series just got saturated into an uninspired,disappointing mess.
I also wanted to add prince of Persia,devil may cry&Castlevania trilogy but they all have a sequel which ruins the trilogy.