Combiners...ahh this is a sticky question. I'm going to devide my answer up into several parts

In the first place, I define a Gestalt, and a combiner as two sepperate concepts. Combiner is a general term that can be applied to anything from Dreadwing's combined aerial mode, all the way to Magmatron's merged beast and robot modes. In other words, and combining at all.
Gestalts on the other hand are a fan dubbed term for a combiner made up of multiple individuals, who combine into a greater whole, in a manner greater than just being hooked together. Devastator for instance, is not simply Hook running the show with everyone else holding on. He has his own (albiet limited) personality, sepperate from any individual constructicon.
Okay, now that we've established what I call a combiner and what I call a gestalt, lets pick my champions...
In the matter of a combiner, I think one of the great unsung transformers of our time is Magmatron. Each of his beast modes is detailed, carrying a freaking Huge level of articulation (How many transformers can count articulation by the Dozen?) and a very nice pair of combined modes. Admittingly, the Dinobeast being a bit of an afterthought...but still a triumph of BW era combiner tech.
As far as gestalts go, the selection is far harder to choose from. I'm not by any means fanatical about the G1 era of transformers, but they had so Many good ones its almost scary...and rather sad that modern combiners really can't keep up. As much as I love my Energon Bruticus and Devastator molds, I don't really feel they're as good as their G1 counterparts when it comes to originality of design.
I think for Gestalts I'm going to have to go with G1 Bruticus, with his Urban Camo Ruination form being my favorite varient of the mold. I have high hopes for his encore version though.
Railracer I never got....he's trains....I just don't get that impressed by an alternate mode thats incapable of going in anything but a straight line. All Megatron would have to do to loose them is move to the right...
BuildKing was okay...I like his constructicon repaint, but his design was just...weird. Didn't really do what I wanted from a construction vehicle set.
Oh, and Devastator get a A+ for being twenty years ahead of the curve with being able to stow his gestalt components in vehicle mode.