Really just Springer, the Seekers, the weaponizers, Megatron, Soundwave, Flywheels, Spinister, Apeface, Astrotrain, Refraktor, the Datsuns, Ironhide, Chromia and Ultra Magnus. Arguably Shockwave, but he's never had a really strong alt mode anyway.
Honestly, the only alt modes I really like are Sideswipe, Optimus, and Hound, kind of. And it's really just that they're not quite as bad as the rest, but they're a far cry from being legitimately good alt modes. I'm an alt. mode fag though. I entirely understand a majority of TF collectors give not many fucks about alt modes and would rather see them compromised to facilitate better robot modes, but to me part of the appeal of a good transformer has always been an alt mode that just looks good on its own, and Siege is REALLY lacking in that department, whether you want to acknowledge it or not. It's partially why I'm so excited for ER Cliffjumper, even with the partsforming, his alt mode is pure sex. Reminded me of one of my old favorites, pic related.