Interesting question, and after sitting down to think about it, here are my top 20 game series of all time:

1. God of War
2. Ninja Gaiden (including the oldschool NES ones)
3. Castlevania (including the oldschool NES ones)
4. Super Smash Bros
5. Donkey Kong Country
6. Devil May Cry
7. Kirby
8. Sonic
9. Guilty Gear
10. Legend of Zelda
11. Super Mario
12. Mario Kart
13. inFamous
14. Assassin's Creed
15. Street Fighter
15. Marvel VS Capcom (tied with Street Fighter)
16. Metroid (I hated the Prime series though, I HATE first person that's why)
17. Breath of Fire
17. Resident Evil (tied with Breath of Fire)
18. Final Fantasy
19. Mortal Kombat
20. Dead Space

1. Metal Gear Solid
2. Jak and Daxter
3. Kingdom Hearts
4. Pokemon
5. Metroid Prime
6. Super Smash Bros
7. Dead Space
8. Ratchet and Clank
9. Fallout
10. Elder Scrolls


Thats all I got for now. First 3 in that order, the last 7 in no particular order.

Dude, you forgot Rachet and Clank.

I can't really come up with a top 20, but I'll give you my top 10.

1. Silent Hill (If anyone didn't see this coming, you deserve a smack on the head)
2. Resident Evil(*puts on hipster glasses* Before it was cool yo)
3. Bioshock
4. Turok
5. Metroid
6. Dead Rising
7. Dead Space
8. Left 4 Dead
9. Pokemon
10. Sonic the Hedgehog

Fixed. Completely forgot about that. I always forget when I do things like this. God TAMMIT

In no particular order because I'd never be able to decide.

1. Resident Evil

2. Mortal Kombat

3. Sonic the Hedgehog

4. Super Mario

5. Super Smash Bros.

6. Assassin's Creed

7. The Bouncer

8. Gauntlet: Dark Legacy

9. Mario Tennis

10.Tekken

11. Soul Calibur

12. Dead or Alive

13. Mario Kart

14. Dragonball Z

15. StarFox

16. Metal Gear Solid

17.Grand Theft Auto

18. Mario Strikers

19. Infamous

20. Madworld

Will add the rest later, dunno what else to add

1. Halo

The only series where I have enjoyed all installments.

Lets see what we can do here.

1: Mass Effect

2: Mortal Kombat

3: Max Payne

4: Gears of War

5: GTA

6: KOTOR

7: Fable

8: Lego Star Wars (I know, I know.)

9: Elder Scrolls

10: Fallout

and that's al for series, to finish the list with standalone games or series I've only played 1 game from...

11: Alan Wake

12: Batman: Arkham Asylum

13: Indigo Prophecy

14: Borderlands

15: Spider Man 3

16: Star Wars: TFU

17: Advent Rising

18: Marvel UA

19: Dead Rising

20: Dragon Age

I don't know if I'm even a fan of 20 series. Here are my favorites

Super Mario
Pokemon
Super Smash Bros.
Fire Emblem
Mortal Kombat
Street Fighter

I've only played one elder scrolls game, and I'm not sure you can call the Batman Arkham games a series, so I guess that's all for me.

Here's My Top 20 off the top of my head (and in no particular order):

1.Mortal Kombat
2.Street Fighter
3.Guilty Gear
4.Tekken
5.BlazBlue
6.Marvel Vs. Capcom
7.Rival Schools
8.Final Fantasy
9.Elder Scrolls
10.Fallout
11.Yu-Gi-Oh!
12.Super Mario
13.Sonic The Hedgehog
14.Metroid
15.Pokemon
16.Smash Brothers
17.Aliens vs Predator
18.Tenchu
19.Zelda
20.Naruto

1. Mortal Kombat
2. Pokémon
3. Killzone
4. Resident Evil
5. Battlefield (3 comes out Tuesday *JIIIIIZZZZZZZZZ*)
6. Need for Speed
7. Marvel vs Capcom
8. Batman Arkham Series
9. Street Fighter
10. Call of Duty
11. Uncharted
12. Sonic The Hedgehog
13. Burnout
14. inFamous
15. God of War
16. Legend of Zelda
17. Super Smash Bros.
18. Crash Bandicoot
19. Mario Kart
20. Spider-Man Movie Games (Mainly the ones that let you free roam)
Every game Before Uncharted is in order, Every game after Call of Duty is not in order.

In no specific order.

1. Pokemon

2. Mortal Kombat

3. Mario (all games, including party and the sports games [exlcuding the olympic ones and wii sports game] )

4. Batman "Arkham" series

5. Call Of Duty (Only MW games and COD 2)

6. Gauntlet: Dark Legacy

7. Sonic The Hedgehog

8. Tekken

9. Saints Row

10. GTA (Excluding GTA IV... It tried being too serious and was a fail)

11. Rock Band

12. Bioshock

13. Forza

14. Fable

15. Mass Effect

16. Crash Bandicoot (First 3 "main" games and CTR)

17. Halo (anything except ODST and Halo piece of shit.... erm... I mean Reach)

18. Amplitude / Frequency

19. Dot . Hack (First 4 parts [The original 4 games] )

20. Kingdom Hearts.

Wow, I'm shocked that I'm the only one so far who loves Devil May Cry enough to put the series in my list. I didn't like the games when they first came out, I didn't really get into Devil May Cry until number 3, but after that game I went back to play the previous entries. Even number 4 was fun for me despite the fact that people hated it. Looking forward to the HD re-releases of the games.

I want the HD remake of MGS and the rumored HD remake of Jak and Daxter.

Castlevania
Contra
Disney/Capcom
Ghouls n' Ghosts
God of War
Legend of Zelda
Kingdom Heats
Mega Man
Metal Gear
Metal Slug
Metroid
Mortal Kombat
Prince of Persia
Ratchet & Clank
Resident Evil
Sly Cooper
Sonic the Hedgehog
Super Mario
Super Smash Bros.
Wario

20?!
Let's see what I can do.
Hmmm.
In no particular order.
Tenchu

Mass Effect

Spyro The Dragon (The First Three)

Crash Bandicoot (first 3)

Mortal Kombat

Halo

Resident Evil

Odd World(Abe)

Hitman

Splinter Cell

Legacy of Kain

Loaded

Soul Calibur(though it is really wearing thin now)

1.Mortal Kombat

2. Megaman ( all of them where good)

3. Darkstalkers

4.Medabots (to bad only a few games where in english)

5.BlazBlue

6.Bioshock

7. God of War

8.Final fantasy

9.Castlevania

10.Rachet and Clank

11 Bleach (the fighting games on the DS and PSP where good but the others ok)

12.Soul Calibur

13. The Legend of Zelda

14. Earthworm Jim

15.Resident evil

16.Call of Duty

17.James Bond (Some where good others werent)
18.Marvel vs Capcom
Sorry only 18 i could think of.

Gears of War
God of War
Resident Evil
Call of Duty
Doom/Quake
Half-Life
Street Fighter
Madden
Mortal Kombat

Pretty much what everyone else before me had.

...oh and Guitar Hero.

In no particular order (and I only have 15)...
-Mortal Kombat
-Pokemon (Generations I and II, and HG/SS)
-Lost Kingdoms (seriously underrated GameCube games)
-BioShock
-Tekken
-The Legend of Zelda (excluding-LoZ:TP)
-Resident Evil (pre-RE4/RE:CV)
-Silent Hill (pre-SH:O)
-Quake (excluding Quake II and Quake IV/Quake Wars)
-Banjo-Kazooie (excluding Nuts & Bolts - never played it, and don't want to)
-Soul Calibur/Blade
-VectorMan (!)
-The Elder Scrolls
-Doom
-Tetris (I love Tetris Blast on GB)

20's a little much for me.

1. Suikoden
2. Star Wars: Knights of the old republic
3. Twisted Metal
4. Mortal Kombat
5. Pokemon
6. Dragon Age
7. Shin Megami Tensei series
8. Elder scrolls
9. Final Fantasy

In Order Of favourite, top being number one. (off the top of my head):
--------------------------------------------

Resident Evil
Mortal Kombat
Socom (only for S2)
Metal Gear Solid
Mass Effect
Silent Hill
Demon/Dark souls
Halo
NHL (EA sports)
Onimusha
Final Fantasy
Soul Calibur
Tekken
Tomb Raider
Killzone
Bloody Roar
Diablo
Legacy of Kain (still unfinished)
Bushido Blade (amazing ps1 game)
Coolboarders (has WAY too much fun on ps1)

I'm sure there are a few I am leaving out and I am gonna kick myself later on. But those are a variety of 20 I could come up with.

1. Mortal Kombat

Don't Hate

Grr, I was gonna make this. But here:
1.) Mario series
2.) Saints Row series
3.) Sonic series
4.) Super Smash Bros. series
5.) Kirby series
6.) Naruto Ultimate Ninja series
7.) Naruto Clash of Ninja series
8.) Crash Bandicoot series
9.) MK
10.) Dragonball Z

In no particular order, excluding the first three:

1. Mortal Kombat
2. Soul Calibur
3. Pokemon
4. Super Smash Bros.
5. Marvel vs Capcom
6. Tekken
7. Dead Or Alive
8. Final Fantasy
9. Kingdom Hearts
10. Darkstalkers
11. Super Mario World
12. The Sims

I can't think of anything else right now. I'll add more later.

Oh shit, one I forgot and which no-one esle seems to have:

Timesplitters! They SO need to make a Timesplitters 4 asap.