That's a tough one. It's early days yet.
The Mek Book rough has 26 fairly common units in it. Most of them atre based on the RPG Meks, though some are new.
The Nordic S-type and ESD-type Meks are pretty unique in concept, and fitr the Nordid defence/deterrence model. The American Combat Vehicles Sniper, with its hypervelocty cannon and Stealth configuration, is pretty neat. The British Huscarl is cheap and... well, it's cheap.
There are other new designs; Halsey's 'Liberator' Mek (actually a Volksmecha-Egide 'Hussar' with a neat black paint job), the old IAI Bastion... some others too. Design philosophy does make a difference now. You want stealth? Fine but you lose capcity. Overbuild and create an unkillable mosnter if you like, but weapons fit will suffer.