60t:
Dragon: Not a beginners mech at all. Suffers from issues with the massive belly/nose hitbox. Pretty fast for a heavy, otherwise flexible loadouts, most of the stuff arm mounted. No JJ's make it difficult to handle in the current meta. Extremely hard to master. Cram in a large XL engine and make a run for your life. While doing, try to make them pay for your hide. If you encounter a Dragon, fire away, you'll hit the center torso eventually... (Now that I think of it, it plays like an extremely fat Jenner without JJ's

)
Hero variant: Fang/Flame: Nothing really special about them. The Flame has a cockpit level mounted ballistic slot which make it a little more popular in the community (it can crest hills).
Quickdraw: A giant martian space robot. Any by giant I mean... gigantic. Seriously, it's legs and arms are huge... With a decent XL engine is goes pretty fast and can still pack some punch. The standard variants are all energy/missile. It's kind of love it or hate it, there's not really an in between. Other than that, it's most remarkable asset is, that you usually don't see them. They're very rarely fielded.
Hero variant: IV-Four: This is actually a special bird. It has arm mounted ballistics and comes stock with an XL 240. Nothing too spectacular, but hey, at least it's still new and so you see at least some of them.
65t:
Catapult: One of the more iconic IS LRM boats. Urgently needs an XL engine. The headshot issue from closed beta is no more... at least Joe Derp has issues headshotting it from across the map. Still a very common sight, tho -C4 and -C1 are getting more and more rare. Nothing too special about the chassis. Just don't get cornered in a Catapult, so try to be as fast as possible (XL300). The -K2 is still popular in Gausscat or Boomcat layout. Either shoot the ears of the side torsos. All but the -K2 have JJ's.
Hero variant: Jester. This IS the bigger, better, meaner, evil, fat brother of the Jenner. It can sport up to 355 rated engines and is literally a -K2 without ballistics but with JJ's. Hilarious in a lazor boat layout.
JagerMech: The end-of-all-days mechs of the IS. Known for high mounted ballistic and energy hardpoints and king of the hill humpers. Everyone tends to say it's newbie friendly... I don't think so. You need an XL engine in there to sport any serious layout. Your side torsos get shot out pretty easily. The easiest way to kill a Jager is to sneak up from behind and shoot out the rear side torso, boom goes the Jager. You really need to be aware of your surroundings and hug your back to a wall. JagerMech usually are priority targets.
Hero variant: Firebrand. One of the most, if not the most popular hero mech there is. Ballistic and energy mounts in the arms... make for some really bad day on the receiving end. Comes stock with an XL280 which is also a nice engine for Catapults and Cataphracts.
Thunderbolt: When it first came out I wondered if they would just copy past the appearance to the Summoner

. And that's pretty much true to the design. It's a pretty tanky mech if you don't equip XL engines. You can make them work, but I didn't like it. Significantly lowers your expected lifetime. Pretty balanced hardpoint layout. The -9SE can mount JJ's. It's a nice workhorse, but really needs those efficiencies unlocked. Hard to find a weak spot on that mech. Try to core it right away.
Hero variant: none.
70t:
Cataphract: Another end-of-all-days mech. Gorilla arms (you can literally tickle your toes with them) make it's life a little harder, but it's a bit more tanky than the JagerMech, while sporting similar hardpoints. Contrary to the JagerMech the arm hardpoints are down below, while the side torso hardpoints are high mounted. Needs an XL engine for the more serious layouts, so shoot at those side torsos. The odd bird is the -4X which can only mount a 255 rated engine. The -3D has JJ's and is renown for it's poptarting capability. Cataphracts get heavily focused as soon as they're spotted.
Hero variant: Ilya Muromets. The goto mech in the Cataphract class. Can sport some ridiculous loadouts (triple UAC/5 or for the lulz triple Gauss). Comes stock with an XL 280.
Summoner: Clan chassis. Shares the same traits with all Clan chassis. The iconic Jade Falcon jumping jack flash. It really suffers from being locked to standard internals, which severly limits it's loadout options. Also it has hardwired JJ's and is significantly underarmored in stock. With the introduction of the Timber Wolf S it looses it's advantage of the JJ's, so it's limited to a niche role. Shouldn't be underestimated tho as it can really ruin your day if it get's a jump on you. Spread out hardpoints make it hard to effectively disarm.
Hero variant: none.
75t:
Orion: The mini-Atlas. Shares the hardpoint layouts with the Atlas, but is more nimble. It appears to be a little taller than the Thunderbolt by sharing some traits. The side torsos seem to be a bit smaller tho. Can take some serious abuse before going down when equipped with a standard engine. XL might work, but I would refrain from using them in an Orion. Orions are usually not the target of choice if there are Jagers or Cataphracts around. The -3M comes stock with an XL300.
Hero variant: Protector. Aleksandr Kerensky's personal ride. It's hardpoints took me a while to get this thing to work, but it's really pretty with those SLDF markings. Nothing too special tho.
Timber Wolf: Probably the most iconic mech of the era. The Timber Wolf is the go-to mech for every Clan pilot. Well spread out and balanced hardpoints, huge engine, ability to mount JJ's, very good hit boxes make this thing incredibly powerful. Sweep the legs, as pilots tend to shave armor from their legs or just headshot it if you can. Otherwise this beast will ruin your day.
Hero variant: none.