I had to dig out all the totes I put my bot in to clean, and properly put away, and it got me thinking, what is my favorite from each TF line, and this is what I came up with,
G1- hard to say, I love the combiners, so its going to be Defensor.
G2- the Lazerods
BW- Magnaboss,(combiner of course)
BM- Blastpunch Primal is good
RID- Omega Prime
ARM- minicons, and the powerlinking gimic is neat on all the bots
ENG- Scorpinock
CYB- Primus
Classics- all of em
M1- not a fan of the movie bots, blaaaaw
Ani- hard to say, so hit and miss
M2- same as above.

 

TFTM- Payload or GTFO  
Animated- Shockwave, even though every single figure in that line was awsome  
Universe/Classics 2.0- Ties between Cyclonus and Hound w/ Ravage
ROTF- The whole HA line

 

Movie 1- Deluxe Class Landmine
ROTF- Leader Class Optimus Prime

 

G1- Bruticus
G2- Laser Prime
BM/W/II/Neo- Inferno
RiD- Landfill
Unicron Trilogy- My personal favorite is probably Cybertron Sideways/Noizemaze
Movie 1- Starscream
Movie 2- HA Bumblebee
Animated- Leader Megatron
Classics/Uni- Voyager Inferno.

 

G1- Shockwave or Jetfire
G2- Tank Megatron or the Laser Rods
Beast Wars- the original Optimus Primal is sheer awesomeness
Beast Machines- Magmatron (that's right, the BW Neo figure is the best   )
RID- Optimus Prime
Armada- Bendy Prime or Unicron
Energon- Galvatron or Omega Supreme
Cybertron- Primus, maybe Hotshot
Movie 1- 08 Bumblebee, Ironhide, or Leader Brawl
Animated- Jet Twins, Ultra Magnus, or Megatron
Movie 2- Optimus Prime...scratch that....EJECTOR

 

Beast Wars-Depthcharge..or maybe rampage...TM megatron too...too hard to choose. :[
Beast Machines- Hammerstrike, ftw
RID-Omega Prime [Technically, that's cheating, but Prime and Magnus together is just too awesome]
Armada-Unicron
Cybertron-Primus [of course]
TFTM-09 leader Prime
TFA-Leader Megatron....no wait, shockwave, no wait...samurai prow...no wait...GAH! They're all so good

 

BW- I want to say Quickstrike, but I want to say Cybershark... But I'm going with Tigatron for being the one that got me into this whole thing. Maybe Tigerhawk.
BWNeo- Corahda
BM- Beast Changer. I like that one for some reason.
RID- Scourge
Armada- Did anything good come out of Armada? Bendy Prime?
Energon- Sharkticon
Alternators- I like me the RX-8.
Cybertron- Am I allowed to say Vector Prime even though I no longer have one but am looking to import one from Korea in the more attractive gray color?
Classics- Megatron (the gun)
Movie 1- Leader Brawl, no questions asked.
Animated- Voyager Bulkhead maybe?
Movie 2- Bludgeon, Thrustaway, ICT?
Universe- Inferno. Cyclonus photographs better but I find it not that great in person.

 

G1- Metroplex
G2- Laser Rods
Beast Wars-Terrible
Beast Machines- also terrible
Machine Wars- Sandstorm
RID- Scourge
Armada- Unicron
Energon- Tidal Wave
Cybertron- Wheeljack/Downshift
Movie 1- Jazz
Animated- Blurr
Movie 2- Scout breakdown
Universe- Inferno
Universe 2.0- Blades

 

BT-18, one of the best.

 

Best toy? All I can come up with is my favorites

G1- megatron
G2- laser Prime
BW- Big Convoy
BM- none
RID- car brothers, JRX
ARM- unicron
ENG- arcee
CYB- primus
Classics- prowl
M1- salvage
Ani- bulkhead
M2- leader prime

 

TFTM - Deluxe Longarm
Transformers Animated - Grimlock
ROTF - Leader Prime

 

G1: I'm forced to go with the original Prime. A great fun set that interacts well with the other Autobots.
G2: Cyberjets. You could release those today as Scouts and they'd still be awesome.
BW: Polar Claw. He fires his leg as a bat.
BM: i neither kno nor care
RID: Prime again.
ARM: Tidal Wave, probably. Only figure that makes me glad I own some minicons, 'cos he looks great covered in a load of aircraft.
ENG: Sharkticon.
CYB: The Cyber Defense Scattorshot mold is fantastic.
Classics: Bumblebee. I think just how good this figure is sometimes gets a little overlooked.
Universe: Hound for two fantastic modes. Honorable mention to Sideswipe/Sunstreaker.
M1: Landmine, I guess. Or possibly the Real Gear subline.
Ani: Sentinel Prime. Only figure that didn't make me go "he'd be perfect if only..."
M2: Ransack. Yeah, Ejector is cool, but Ransack can stand up.

 

- G1 - Powermaster Optimus Prime w/Apex Armor
- BW - Transmetal Terrorsaur
- BM - Jetstorm (deluxe)
- RiD - Omega Prime
- TFU 1.0 - Razorclaw
- Armada - Unicron
- Energon - Bruticus Maximus
- Cybertron - Vector Prime
- Classics - Jetfire
- TFTM07 - '08 Camaro BB
- TFU 2.0 - Cyclonus
- Animated - Shockwave (grey)
- RotF - Bludgeon

 

G1- Devastor
G2 - F*ckin' Blue Grimlock
Beast Wars - ?
Beast Machines - ?
RID - Ultra Magnus
Armada - Unicron
Energon - Inferno
Cybertron - Primus
Classics - MP Skywarp
TFM1 - Prime (Leader)
Animated - Megatron (Leader)
ROTF - PRIME (Leader)

 

G1: Optimus
G2: Dreadwing
BW: TM Rattrap, Ultra Class Optimus Primal
BW 2: Galvatron
BW Neo: Big Convoy
BM: Jetstorm (Deluxe)
RID: Optimus
Armada: Unicron
Energon: Starscream (G1 repaint)
Cybertron: TIE Crosswise, Vangaurd/ Cybertron Defense Hot Shot
Classics: TIE: Optimus (Voyager), Megatron (Voyager)
Movie 1: Wreckage
Universe: TIE Cyclonus, Hound
Exclusives: BW Metals Ravage,
Henkei: TIE Thrust, Dirge, Red Alert
ROTF: Baron Von Ransack

 

G1: Soundwave
Classics: Optimus Prime - Voyager
TFTM: Arcee
Universe: Inferno
Animated: Stealth Lockdown
ROTF: HA Bumblebee

 

G1: Seekers, any.
G2: Dreadwing
BW: TM Tarantulas
Armada: Unicron
Energon: Mirage
Cybertron: Metroplex, Voyager SS
Movie 1: Wreckage
Universe 1: Black Convoy, Night Slash Cheetor
Classics: Stormcloud
Henkei: Onslaught
ROTF: Long Haul

 

G1: Devastator
G2: Cyberjets
BW: Optimal Optimus
BM: Blast Punch Optimus Primal
RID: Optimus Prime
Armada: Deluxe Optimus Prime
Energon: Rodimus
Cybertron: Leader Optimus Prime
Classics: Deluxe Starscream
Movie 1: Deluxe Protoform Optimus Prime
Universe: Onslaught
Movie 2: Leader Optimus Prime. Honorable mention to Deluxe Swerve.

 

G1: Optimus Prime
G2: No opinion
BW: TM Optimus Primal
BM: Jetstorm (deluxe)
RiD: Scourge
Armada: Unicron
Energon: No opinion
Cybertron: Hot Shot
Classics: Mirage
Movie 1: Optimus Prime
Movie 2: Optimus Prime
Animated: Blurr

 

G1- Predaking
G2- I'm kinda fond of the Dinobot repaints
RID - Omega Prime
Armada - Seekers
Energon - Scorponok
Cybertron - Sideways
Classics - Jetfire
Alternators - Meister or Hound
Masterpiece - Optimus Prime
Live Action Movie - Leader Prime
Universe - Cyclonus
Animated - Leader Megatron
Live Action Movie 2 - Leader Prime hands-down now, but maybe Leader SS