Some of the mechs and Builds I recommend to newbies:
For the Clans:
The GaussBringer
Fantastic at med -> long range, decently fast, ECM, reasonably survivable. Even if they suck, the ECM is still beneficial to the team.
GaussVomit FTW.
Being an omnimech, you also aren't locked into sucksville with poor mastering variants like you are with some 'Sphere mechs.
For the 'Sphere:
BlackJack
I advise heavily that this one requires more caution, but it's a great mech to come to grips with a poking support playstyle, which IMHO is one of the better ones for new players to adopt. It also teaches about protecting fragile mechs, as the XL is like dynamite if you aren't careful.
If they find themselves getting rekt, they can drop speed and gain survivability with a STD engine.
Honorable mentions go to the ACH, the MadCat (though it's less newbie friendly than other Clan mechs), the SCR, and the Stalker.
I also tend to advise against
pure LRM boats, as their effectiveness decreases exponentially as you move up tiers, and direct fire weapons are something everyone should practive with.