One mech that often gets thrown around is the Shadow Hawk. Its quick enough that you can get a bonus for movement, so it rewards a mover, yet its sturdy enough that it dosen't excessivly punish someone who can't move well, its got one of each type of weapon (AC, laser, LRM, SRM) so you can learn the rules there, and its easy on heat. For me, its nice to start of with an easy to use mech, if that's a Shadow Hawk or a Mad Dog C or Panther or what ever, and get the basics out of the way. Movement, shooting, taking damage. Heat is a more advanced concept, if only just very slightly more. Do it after a game or two. Do very high speed after a game or two. Do lack of speed, assualts, only after you've demonstrated speed, or I think it can become a crutch to learning movement.
It depends a lot on the player. It depends a lot on the group. And it depends on if you have time to do several short games, or if you need a trial by fire and ok, here's your Tessen, watch out for ECM and try and get that iNARC and TAG in play so we can support you with LRM fire. So long as it works.