I'd get them all. I have TM Tarantulas on the way but the other 2 I have owned and currently own the Armada repaint of TM Megatron. TM Megatron is quite fun and so is TM Rampage, right now I'd have to go with Rampage just because he's total win.
TM Megatron is an excellent figure and I'd highly recommend him. However, IIRC, the toy can suffer from Gold Plastic Syndrome. Rampage is huge and the beast and vehicle modes are great...plus he comes with the massive gun. On the down side, his torso doesn't lock together well in robot mode and there's a lot of kibble on the lower arms in that form. TM Tarantulas is neat in all three modes and, of the three listed, probably my favorite overall. In my experience, none of these three are prone to the kind of chrome flaking that appeared on TM Rattrap.
I rarely handle my rattrap i only transformed him about once or twice so i hope it doesnt chip or anything.
Of those three, Tarantulus is probably the best "Bang for the buck" in terms of looks and quality for the price. TM Megatron is actually my favorite, but that toy unfortunately does not age well, and the dark brown plastic is prone to becoming extremely brittle and breakable ("Gold Plastic Syndrome"). The Japanese "BW Metals" version might be better in that regard, if you can find one for an OK price. Rampage is really cool too. Really there's no wrong answer here.
Megatron and Tarantulus are both excellent figures, but my favorite is actually TM Terrorsaur. I dunno, something about double-jointed knees and ridiculously oversized arms really did it for me. Of the 3 you mentioned, I'd pick Tarantulus.
i actually like TM Waspinator too, it was my first beast wars figure as a kid and i loved it. not one of the most popular figures though.... but rampage is totally awesome. hes got a great size to him that just dominates other figures in beast wars. i liked him a lot when i had him. i wish i had gotten depth charge to face off, never did though...
Metals Rampage has an amazing beast mode, but his bot mode is too kibbletastic for my tastes. That's why I'm considering selling him.
They're all excellent. Megs can suffer from gold plastic syndrome and his head sculpt isn't that great though. Plus his cutlass arm thing is a bit naff. I recommend getting the golden disks + reader upgrade set for him. Rampage is awesome due to his sweet gun and imposing size but he's a bit fiddly. Tarantulus is solid and good value.
