Okay, I'm gonna go through my collection and pick a stand out alt mode from each year. Again, I'm particularly drawn to modes that look or work well as robots in disguise.

1984: Soundwave. Realistically scaled radio microcassette player where the AA batteries even become the weapons! Also, 5mm posts FTW.
1985: Downshift. What a silly-car is the Celica.
1986: Ultra Magnus. Okay, the trailer is unrealistic, but the interplayability with other Autobot Cars is just too much fun. It was always so cool making him haul other Autobots to battle. It's a shame that the scene of Ultra Magnus doing this in the Battle of Autobot City was deleted from Transformers The Movie.
1987: Goldbug. Finally, a realistically proportioned Volkswagen Beetle.
1988: Nightbeat. Okay, the chopper flames on the side of a blue Porsche 959 aren't standard, but also not as conspicuous as the racing decoes on Jazz!
1989: Swindler. Great Scott, he's a DeLorean! Honourable mention to Greasepit for his service station base!
1990: Big Daddy. A 1957 Chevy, and that name!
1991: Joe, the Super Hitachi Express train. I've been on this train a few times in Japan.
1992: Discharge, the cute little fire engine with the funny name.
1993: Optimus Prime. Yeah, I don't own too many toys from the G2 era (my collecting slowed down during this time), but hey, it's a Freightliner FL86, it works. Ignore the huge stickers on the sides of the trailer.
1994: Skram; Chevrolet Corvette Stingray with exposed engine.
1995: Frenzy. Another Porsche 959 only without attention-drawing chopper flames. And yay, another Decepticon on this list!
1996: Insecticon. Yeah, a lot of the early BW toys had realistic looking beast modes, but a beetle would be able to sneak about human society without being noticed (assuming that he can mass-shift down to the size of an actual insect). And remember that pre-cartoon BW continuity was set on modern day Earth, so my BW picks are based on this assumption.
1997: K-9. As mentioned before, a German shepherd dog. A popular pet and wouldn't look out of place.
1998: Big Mos. Don't forget the Aeroguard.
1999: Mach Kick. No horsing around.
2000: Super Fire Convoy; Hino Branbilar fire engine.
2001: X-Brawn; Mercedes Benz ML320
2002: Crosscut; Honda City R hatchback. Yeah, I know I'm picking a Diaclone mould that was basically reissued as a Transformer character, but 2002 was a crap year for TF collecting with bugger all new toys released.
2003: Binaltech Lambor; Dodge Viper SRT-10
2004: BT Jazz; Mazda RX-8. Zoom-zoom. Special mention to Hound/Swindle, Jeep Wranglers.
2005: BT Overdrive; Honda S2000.
2006: Alternator Mirage; Ford GT. And one of the few Alternators that I consider superior to its BT counterpart.
2007: Speed Dial 800; mobile phone. We all have one. Special mention to Frenzy.
2008: Stockade; modified Cadillac Escalade SUV.
2009: Alternity Bumblebee; Suzuki Swift. Honourable mention to Ejector the toaster.
2010: Reveal the Shield Megatron; Walther P38 - not every day that Hasbro makes a gunformer that they can legally sell. Plus it's compatible with MP Jets!
2011: Wheelack; Mercedes Benz E550. Honourable mention to Soundwave, but the E550 is less attention-drawing than the SLS AMG.
2012: MP Lambor; Lamborghini Countach - yay, HasTak got the Lamborghini licence back!
2013: MP Streak; Datsun Fairlady 280Z
2014: MP Bumble; Volkswagen Beetle Type 1 - yay, HasTak got the VW licence back!
2015: Offroad; utility truck
2016: MP Ironhide; Nissan Cherry van
2017: Sqweeks; beat up Vespa motor-scooter
2018: Cogman; Aston Martin DB11 and the only Headmaster car with both a driver and passenger seat