this one is easy for me. the WFC car mode for Bumblebee is my favorite Transformers car mode, ever. i want them to revisit that, so badly, because I hate the robot mode's proportions.

 

I think Soundwave as a Predator drone is the best modern interpretation of Soundwave and my favorite non-classic alt mode. I wish the arms of the figure were a little more poseable so you can match his TFP stance better, I can never get him to stand quite as ominously as he does in the cartoon.

Thrilling 30 Megatron as a bomber is great, and my #2 non-classic. A bomber makes sense; he gains speed and maneuverability, looses a bit of direct damage without the cannon of his tank mode, but still packs a massive punch as a bomber. The figure even looks amazing and makes for a very imposing Megatron.

Shockwave's WFC:Siege mode is pretty damn awesome looking, and my #3. It's probably my favorite spaceship alt-mode ever.

Runner up, I actually appreciate what they tried to do with making Blast-Off a jet instead of a shuttle for Combiner Wars (I know they did for FoC too, but that one fell flat for me), it makes way more sense with the Combaticon theme. Still prefer the shuttle version for my Bruticus though!

Bumblebee as a Camaro has become so iconic, I have to give it an honorable mention, even if I'm more happy to see him back as a Bug finally.

 

Someone mentioned Arcee being a motorcycle... I do prefer Prime Arcee but that's more to do with the way the character was portrayed in that series than what she transformed into. It's not even the first time she was a motorcycle as this happened before in Energon, 2007 movie toy line, and RotF but I never really liked any of those. I mean of the three Energon looked the most like her name sake despite turning into a completely different vehicle but she's just as boring a character as the G1 version too.

Anyway that's why I didn't mention her myself because my preference for Prime Arcee has nothing to do with her vehicle mode.

A couple non conventional alt modes I forgot to mention though...

Starscream as a tank, Hot Rod as a Helicopter, and Optimus Prime as a Jet. These are figures that really should not have worked but some how do. It's probably because they're all triple changers who still have a more traditional alt mode as well as these non-traditional alt modes. This might be an unpopular opinion but I would really like to see more characters reimagined as triple changers. I kinda enjoy the blend of both a more traditional look and something totally new.

 

Megatron as a helicopter. That's my favorite altmode for him. A shame it almost never happens.

 

Megatron as a tanker truck in DotM.

 

Thrilling 30 Bumblebee turns into a Ford Mustang.
Generations Scourge turns into a Boeing X-48.
Prime Arcee turns into a blue motorcycle.
Generations Kup turns into granddads old pickup truck.
Thrilling 30 Sandstorm turns into a VTOL aircraft and dune buggy.

 

I really liked the TR triple changer Optimus and Megs, also Animated Megatron's helicopter mode was bad ass. Soundwave's and Blaster's noncombat military vehicles alt modes from FOC were perfect.

 

Ironhide as a pickup. Really needs a better toy.
Onslaught as a SWAT APC. Just found out about that toy, and I NEED it.
Grimlock as an actually threatening T-Rex, as opposed to his dumpy G1 POS Rex.
More modern vehicles in general.
Megatron and Shockwave as ANYTHING other than guns. Battlemaster versions would be cool though.

 

Optimus as a fire truck and Ironhide as a humvee have always been just about perfect in my mind.

 

That was Ratchet... Ironhide was pickup truck.

 

I think I would have to echo what some said with Megatron being pretty much anything else other than a hand gun. I prefer him having a tank mode but his double prop jet mode from Animated was also pretty interesting. But a powerful Decepticon like Megatron, it just makes more sense for him to have a powerful alt mode instead of shrinking down and relying on someone else to hold him to be effective.

And going from Animated, I have to admit, I really love the idea of Wreck-Gar being a garbage truck instead of a motorcycle. Based on his name and faction, it just makes more sense and also would be fun to see him as a garbage truck again.

 

This is pretty obscure, but G2 Powermaster Ironhide was actually a Humvee.

 

I stand corrected... also how that a Powermaster? It doesn't appear to have that gimmick. Based on the way the legs look he kinda resembles a Throttlebot but based on the fact that he has actual arms instead of panels sticking out of his sides pretending to be arms he can't actually be a throttlebot.

- = one quick wiki search later = -

It appears there were TWO groups of Powermasters. The guys that were called Godmasters in Japan with the engine dudes similar to Headmasters and Targetmasters. This is what I initially thought of when you said Powermaster.

And these guys I've never heard of before that are essentially Throttlebots but larger.

Also for the sake of clarity... According to the Wiki only the engine guys are called Powermasters. The larger Throttlebots are Power Masters. It actually seems to make a difference here if it's one word or two because the different groups spell the name differently.

 

Prowl in the IDW comics turning into a police suv. i had thought of that idea and then they did it in the comics.

It'd help again to keep him and Barricade from being too samey.

 

Bumblebee as a mini SUV. It still fits his squat g1 nature while making his chest not look as bulbous with the bug roof

 

Optimus Prime as a fire engine.
Prowl as a Police Interceptor Utility vehicle.
Ironhide as a pickup truck.
Bulkhead as a S.W.A.T. van and an SUV.
Omega Supreme as a Cybertronian ship.

Megatron as a tank and a helicopter.
Galvatron as a Cybertronian jet.
Soundwave as a stealth bomber.
Shockwave as a Cybertronian jet and a tank.
Barricade as a police car.