Some good advice here, might as well add my share since i've mastered bolt, phract, jager and onion:
Thunderbolt is probably the best bet. It's probably not gonna get nerfed further, and even then it would survive a small nerf after those huge - if necessary - buffs. MPL variant is a great brawler, unusually fast and powerfull for an IS heavy without sacrificing too much survivability. For long range engagements there is a solid ERPPC variant with high mounted guns. Otherwise there is a more run of the mill LPL variant with less damage but more range, which can also run some nice LPL/ML/SRM brawler setup.
Cataphract has a whole bunch of good builds, although they aren't exactly high level meta - then again, what is. Very solid mech, personally i'm missing something special to be really interested, even after mastering. Notably the arms make very good shields and the two torso energy slots are very high. ECM variant will be definitively interesting tho!
Jagermech can be fantastic, I love mine! Fragile, although not as fragile as you might think, it allows a nice range of support playstyles and the hitboxes and small size (the 'large' shields alone have the circumference of a cata) even allow some combat survivability and limited brawling. JM6-DD is one of the best support fire mechs in the game, 3xUACs+quirks (never underestimate the low jam) are an insane amount firepower if you can get the hits. Most of my jagers use UACs to some degree combined with varying degrees of laser support for alpha/dps styles, but there is ofc the quite dangerous meta-gauß sniper build. Missile-jager also allows some fun, if tricky SRM builds. People allegedly also use them for missile boats (heretics!), otherwise there are - imo boring and unimpressive - AC40 builds.
Size of the paragraph probably shows my bias. I found the mech, while relying on certain build rules, to allow lots of diversity in your playstyle, in order to bringing the huge amount of firepower to the right places at the right time.
Catapult is mostly used for missile boats, while the arms are just too vulnerable for most close combat (especially the 6 missile variant), and the K2 can be used for solid PPC builds with ACs.
Do not make the mistake of simply writing of the cata, tho. Rumors are PGI is planning to revert the terrible arm changes, which might breath some more life into this iconic mech.
Orion is a two edged sword. Can be dangerous when used correctly, but is outshined by more specialised mechs, and the timber outdoes it in every regard (if in a generally more boring fashion).
I still like it, even if it's not exactly a monster.
Edited by Averen, 10 June 2015 - 12:19 PM.