My fave personally is G1 Bruticus, but ever since I got Predaking....it's a tough choice. Very awesome combiner.
Railracer, Energon Constructicons and the Universe repaint Constructicons. Railracer just works, and is more than the three occassionally dodgy modes would imply. Energon Constructicons cos they don't need the Energon parts to work, and its very posable and stable unlike the other two... Universe Coinstructicons look great, solid moulds, and again the multi mode combiner just works for me.
I'll always have a warm spot in my heart for (G1)Superion but of those I've been exposed to I'll have to say it's (G1)Predaking.
You guys ought to check out the Brave series for some unique combining robot designs. Just sayin' For Generation 1, either Liokaiser, Predaking or Piranacon for me. For modern series, Rail Racer, Landfill, and Magnaboss.
the best combiner toy official received from has/tak would be Rail Racer. (I don't consider any prime upgrades/combinations to be "combiner teams") the most "iconic" combiner, would be devy, no doubt. My personal favorite would be Liokaiser.
Well, I think you could make an argument for Cybertron Optimus. Wing Saber's backpack mode can attach to the regular Super Mode backpack's straps. This allows Optimus to combine with Leobreaker and WS at the same time, making a three-bot team. Even better if you add Universe Leo Prime/Lio Convoy to other side. Four together.
Magnaboss, Rail Racer, Omega Prime, Jet Convoy w/ Overload, Armada Perceptor, Superlink Frenzy. And this:
Legends ROFT Devvy is my favourite for being an inventive take on combiners and is a great toy. G1 Bruticus and G1 Computron. Again, great toys and great aesthetics. FP Bruticus for all round win on improving a mediocre base.
Rail Racer would probably be my top one overall; good altmodes, decent (for the most part) robot modes with excellent articulation, and a combined form that looks awesome, has great articulation, and uses all the individual 'bots weapons, so no extraneous parts. Magnaboss and Tripredacus are very cool for the same sorts of reasons. On a G1 tip, I've always loved the Bruticus moulds, especially after the retool used for Ruination, which again manages to use pretty much everything in his combined mode, which is always a plus for me. Also Landcross, and I'm sure I'd love Liokaiser to bits if I had more than just the four Rescue Force redecoes.