Pretty much all of them. I love the fact of, having enough money, you can go purchase your own full size Autobot replica.

 

I'm about 99% sure you could do that with almost every genre of Transformers and it would'nt cost you an arm and a leg...



Anywho, personally i think the best Alt modes were; Frenzy's radio mode, Ravage's alt mode, Laserbeak's alt mode and Alice's alt mode.

Frenzy provided us with functionallity - even his alt mode could be used as a form of disguise that could pretty much be anywhere.

Ravage's alt mode is designed for function also invoking speed and stealth above anything else.

Laserbeak's alt mode (bird) is designed for flight, it also is designed for silent aerial attacks - given little to no sound is heard when Laserbeak flies.

Alice's alt mode is perfect for blending in with humans - with most of her being bio-mechanical allowing her to blend in more along with perform human acts with little to no suspicion

 

Barricade's police alt

 

Brawl is my all time followed by Blackout (Vortex, as I like to call him) and the NASCAR Wreckers.

 

Sideswipes corvette mode (ROTF), Megatron's Tank and Truck, Starscream (all), Dino (cuz i love Ferrari), and Jetfite (gotta love the blackbird)