I can't pick one favorite, but I'll just ring off a few from different lines that I own.
Armada: Megatron and Tidal Wave, Demolishor is great too! I say that you should absolutely pick up Megs and Tidal Wave. The latter is fantastic on his own, all massive and imposing, and he's got that combining gimmick...yeah, leave Tidal Wave on his own. XD. Starscream is also a must.
Energon: If this line did one thing right, it was the combiners. They were by no means perfect, but Steamhammer and his buddes are just fantastic. The crane truck makes a fantastic arm with a big claw-hand, double hinged elbow, and a bicep swivel. Plus Steamhammer has a waist swivel even in combined mode! And that head? Awesome! That lightpiping? The best thing ever!
Cybertron: I really like Wreckloose, he's got this nifty assymetical robot mode. Again, Stascream is a MUST. If you have room for SupremeScream, get him.
Movies: I really like Dotm Air Raid, he's just fun to play with, and you can probably still did him for cheap. Dotm voyagers Megatron and Shockwave are quite good too. Wreckage is okay, but he can't look straight forward, it's really bothersome.
CHUG: Oh my LUGNUT. I'm sure I don't need to say much with all the Lugnut love of the past few days! Also RTS Legends G1 Megatron is quite good.
Prime: Vehicon, Vehicon, Vehicon!!!

. Also Knock Out is quite fun, just don't pay too much for him, he's tiiiny. If you have a Target or Walmart near you, you might be able to find him, and get him for about $7, I know my Taget still has overstock ( a little at least) from the last wave of PRID deluxes.