1) Super smash bros melee, seriously i loved this game as a kid and now i'm older i'm still making an effort to get better and hopefully get into the competitive scene a little bit after watching the smash documentary a year or 2 ago (which i would highly reccommend doing). There's an absolutely obscene amount of depth and the skill gap is super high considering how quickly and accurately you need to press buttons.
2)Halo 3 - definitely the single video game i've played most. I have no idea what the stats are but it wouldn't surprise me at all if i've got a couple of months game time for good ol' h3. The multiplayer is something else - i feel like fucking superman when i'm properly zoned in to montage mode and destroying all teh n00bz
3) Pokemon crystal - Best pokemon game ever full stop.The music, the region, the actual pokemon themselves and if all that wasn't enough you can even go back and do kanto aswell! also the fact that the sprites moved when you initially sent them out blew my mind as a young child, that shit didn't make a return until like 5 years later in pokemon emerald and even then it wasn't as good.
4)Pokemon Colosseum - Playing pokemon on console in 3d was sososososo cool back when this came out. Damn i just love the nintendo gamecube way too much.
5) Half life 2 (and all the episodes) - idk i was kinda struggling to pick out a 5th game that stands out as much as the above to me. I've recently replayed half life 2 and it's forsure one of the games that's scared me the most. The headcrabs really genuinely creep me out, they make me jump and shit when they jump for your face and those big zombies with the 3 black headcrabs on them are horrific, they always take me by surprise. Idk half life 2 is just an all round really solid and engrossing singleplayer game.
Honourable mentions go to: Pokemon ruby, metal gear solid 1, Crash 3: Warped, all the spyro games for ps1, dungeon keeper for pc (check it out it's actually fucking sick), Crash team racing, animal crossing wild world and Call of Duty 5 and black ops 1 zombies, oblivion