I wouldn't agree with the atlas RS choice - it's a good mech in isolation, but given that the DDC exists and ECM is so important, I would have a *very* hard time recommending any other atlas. The DDC's third missile hardpoint is already a strong advantage, and ECM just puts it way over the top.
That said, I don't think I'd recommend an assault at all for a 'noob mech', as they are deceptively difficult to play *well*, along with being too expensive to both buy *and* trick out using just the cadet bonus. And with the way the matchmaker works right now, every inexperienced or subpar assault on your team is another chance for the enemy to get a tricked out stalker or DDC. It's the same for other weight classes too, of course, but the effects are magnified for assaults.
If I had to pick exactly one mech to recommend for a first buy, it would absolutely hands down be the hunchback 4SP. There really aren't any downsides to the choice. It's a medium, so 90% of the time you aren't giving the other team an ECM mech (in my experience ECM cicadas are much less common than the other ECM mechs). It's on the cheap side, so you could buy it after just a few games given the cadet bonus. Even *completely stock* it's not bad, so if you wanted to you could put the rest of your cadet bonus towards a second mech. It gets *very* good when tricked out, whether you go for large lasers or stick with mediums, upgrade the engine or stay stock, etc - about the only way to cripple it would be to put an XL engine on it. And it's a great mech overall - good array of hardpoints, no super-heavy ballistics to drag you down, no super-vulnerable concentration of 2/3rds your firepower on one bulky shoulder, flexible arm mount lasers on a platform with incredible torso twist, etc, etc. The only downside I can think of is that it might spoil them for other mechs.
Apart from that, I might suggest a cataphract 2X, mostly because it's got some of each kind of hardpoint on a pretty decent base chassis. You can experiment with all sorts of weapon types and builds, it doesn't require an expensive XL engine, and again it doesn't give your enemy an ECM mech. It *does* possibly give them a tricked out phract or cat, though, so it's not as safe as taking a medium. The centurion-A also has that diversity of hardpoints, though it's considerably harder to fit something to all of them given its lighter weight. That three-rack of missiles is mighty nice, though.
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