Probably the Seasons of 24. I always pick them up cheap on Black Friday.

Movie wise, Lord of the Rings. For Star Wars I have each of the movies (except the non-anamorphic OT) and the recent animated series.

I have 11 Friday the 13th movies if you count Freddy v. Jason, and a big metal Paul Champagne case to store them in. I think that's the big one for me.

if you're going by number of discs, as some people are, then I guess it would be the Simpsons at 30+ discs.

I started my Degrassi collection with the 'Junior High' Box set....

Highlander : 51
Hercules : 46
Babylon 5 : 41

Seinfeld - 24 Discs
LOTR - 21 Discs
Superman - 14 Discs
Matrix - 12 Discs
Alien (Including AVP) - 11 Discs
Star Wars - 10 Discs
Dead Like Me - 8 Discs
Arrested Development - 8 Discs

Clint Eastwood collection -close to 50

Where's the easiest way to find out how many DVDs are in each set? I'm not going through my collection and counting each set.

In rough order:

I've got 12 seasons total of Buffy + Angel
Nine seasons of The Simpsons
All seasons of Xena, plus the individual dvd release of the finale.
All seasons of Homicide, plus Homicide the Movie.
Six seasons of Gilmore Girls
All seasons of Babylon 5, plus The Gathering/In the Beginning disc
Six seasons of Seinfeld
All Monty Python TV episodes and movies (but NOT the "best of" DVDs they repackaged recently)
All seasons of Six Feet Under
The Complete Dick Van Dyke set
First four seasons of The X-Files, plus the movie
All seasons of Moonlighting
Four seasons of The Mary Tyler Moore Show
Four seasons of The Bob Newhart Show
All 19 individual disc releases of Lexx
The Avengers Emma Peel Mega Set

Stargate for me :

6 Seasons of SG-1
2 Seasons of Atlantis and
2 copies of the movie.

Buffy The Vampire Slayer: 78 (39 for the individual R1 sets and 39 for the Limited Edition UK box set)

The Simpsons - 9 Seasons
Seinfeld - 6 Seasons/5 Volumes
Star Wars - 6 movies
24 - 5 Seasons

24 all 5 Seasons, South Park Seasons 6-9, The Simpsons Seasons 2-9, Seinfeld all 8 Seasons, Hajime No Ippo (Fighting Spirit) all volumes and Naruto all three uncut box sets.

Puppet Master (boxset, Legacy + vs Puppet Master) 9 discs
Spider-Man films (1 DE, 2 SE, 2.1 + Walmart bonus disc) 8 discs
Aqua Teen (Vol 1-4) 8 discs
Romero's Living (Dead Night ME, Dawn UE, Day 2disc, Land) 8 discs
Nightmare on Elm St (boxset + JvF) 8 discs
Peanuts (2 films, holiday boxset + 2 others) 7 discs
Reno 911! (seasons 1-3) 7 discs
Spider-Man 90's (5 Buena Vista + 2 Sam Goody) 7 discs)
Evil Dead (1 & 2 Book of the Dead, 2 tin, 3 LE) 6 discs
Leprechaun (boxset + Back 2 Da Hood) 6 discs
Spider-Man 67 (complete series) 6 discs

Star Trek - 218. (This includes 3 of the Bonus Disks)

quickly thinking about it:

LOTR
bond ultimate collection v1 and 2 and casino royale.
superman

tv sets: smallville, 24

Is that everything? The 5 series, animated, and movies?

I didn't count all of my Hitchcock movies. Not quite sure about counting number of discs, but it kind of makes sense, except for the farscape DVD-18's (is that 2 discs?)

Summary:
Star Trek - 221
X-Files - 58
Stargate - 52
Twilight Zone - 47
Babylon 5 - 40
Farscape - 38
The Simpsons - 35
Smallville - 29
Seinfeld - 28
South Park - 28

*Movies Only*
James Bond - 23
Lord of the Rings - 18
The Matrix - 16
Evil Dead - 10

TV shows + Movies
Star Trek (221)
- Movies (10*2 discs = 20)
- TOS Season 1 (8)
- TOS Season 2,3 (2*7 discs = 14)
- The Animated Series (4)
- TNG Season 1,3,4 (3*7 discs = 21)
- TNG Season 2 (6)
- TNG Season 5-7 (7 + Bonus Disc = 8*3 = 24)
- DS9 Season 1 (6 + Bonus Disc = 7)
- DS9 Season 2 (7 + Bonus Disc = 8)
- DS9 Season 3-7 (5*7 discs = 35)
- Voyager Season 1 (5)
- Voyager Season 2-7 (6*7 discs = 42)
- Enterprise Seasons 1-3 (3*7 = 21)
- Enterprise Season 4 (6)

The X-Files (58)
- Season 1-4,9 (4*7 discs = 35)
- Season 5-8 (6)
- Mythologies (4*4 discs = 16)
- Fight the Future

Stargate (52)
- Movie (2)
- SG-1 Season 1-9 (9*5 = 45)
- Atlantis Season 1 (5)

TV Shows Only
The Twilight Zone (47)
- Season 1,4,5 Definitive (3*6 = 18)
- Season 3,4 Definitive (2*5 = 10)
- 80's Season 1 (6)
- 80's Season 2&3 (7)
- 90's (6)

Babylon 5 (40)
- Season 1-5 (5*6 discs = 30)
- Movie Collection (5)
- Legend of the Rangers (1)
- Crusade (4)

Farscape (38)
- Season 1 (Starburst) (6) (DVD-18)
- Season 2 (Starburst) (6) (DVD-18)
- Season 3 (Starburst) (12)
- Season 4 (Starburst) (12)
- Peacekeeper Wars (R2) (2)

The Simpsons (35)
- Season 1 (3)
- Season 2-9 (8*4 discs = 32)

Smallville (29)
- Season 1-4 (4*6 discs = 24)
- Season 5 (5) (HD)

Seinfeld (28)
- Seasons 1&2 (4)
- Season 3-8 (6*4 discs = 24)

South Park (28)
- Seasons 1-9 (9*3 discs = 27)
- South Park Movie

Movies Only
James Bond (23)
- Collection 1-3 (20)
- Never Say Never Again
- The James Bond Story
- Casino Royal (BR)

Lord of the Rings (18)
- FOTR (2)
- FOTR: EE (4)
- TT (2)
- TT: EE (4)
- ROTK (2)
- ROTK: EE (4)

The Matrix (16)
- The Ultimate Matrix Collection (10)
- The Matrix LE
- The Matrix (Original Release)
- The Matrix Reloaded (Original Release) (2)
- The Matrix Revolutions (Original Release) (2)

Evil Dead (10)
- The Evil Dead: CE
- The Evil Dead Book of the Dead
- Evil Dead II Tin
- Evil Dead II Book of the Dead
- Evil Dead Trilogy (R2) (4)
- Army of Darkness: DC
- Army of Darkness (R4)

Stargate - 48 discs

M*A*S*H Seasons 1-11
Twilight Zone - Seasons 1-5
James Bond - Ultimate Editions 1-4