>>8250601Start with the MSG Movie Trilogy. Good, self contained story that cuts out all the fluff and gives you a taste of the UC timeline - the main part of the franchise. Think of it as OT Star Wars.
Alternatively you can start with one of the AUs (e.g. G, SEED, 00) but know they are one-offs and don't necessarily represent the rest of the franchise.
And if you're just in it for the toys and designs, watch Build Fighters ;)