Best Robot Mode: MP10 Convoy; looks good, loads of features and heaps of fun to play with
Best Alt Mode: Alternity Bumblebee
As I said in my Cybertron Collectors video, one thing that I find endearing about G1 Bumblebee is the unassuming alt mode that epitomises being a robot in disguise. Flashy sports cars and race cars are more eye-catching and less useful as disguise modes since they're more likely to draw attention.
A little yellow Suzuki Swift works really well, as these are cars that I commonly see on streets. And yeah, it's not a unique thing - there are plenty of other Transformers out there with unassuming alt modes that work well as disguises too (including most BTs and Alternities). Even in early G1 we had toys like Trailbreaker, Hoist, Ironhide, Bluestreak, Hound etc.; all vehicles that wouldn't look out of place parked on a residential street or in a car park. I really like Bumblejumper too - the good ol' Mazda 323! Heck, this morning I saw a Ford Mustang GT drive past me and I immediately thought of BT Grimlock/Wheeljack.
As far as beast modes are concerned, my favourite would be K-9. A German shepherd dog would easily blend in with human society (and pre-cartoon BW canon was meant to be set in the modern day!). Insect and arachnid modes are also pretty cool if they can mass-shrink down to the size of actual bugs, as the G1 Insecticons did in the Marvel comics (and same with ROTF Insecticon). This allows them to infiltrate practically anywhere. And the Autobot Pretenders in Japanese G1 also worked well as they could mass shrink down to human size and walk around wearing human clothes. Seriously, being able to disguise yourself as a human is pretty damn handy on Earth. Although technically these are shells, not alt modes -- but ROTF's Pretender Alice had a human alt mode (although her transformation and robot mode were bollocks). It's weird how Anglophone G1 continuity never tried mass-shrinking the Pretenders and so their disguises only worked once against some very naive/stupid Decepticons, and after that only on off-world locations inhabited by giant human-like aliens (e.g. Femax, Pz'Zazz, VsQs etc.).