The Longinus offers good mix of firepower and range. The David Gauss Rifle config isn't powerful but has reasonable range, while the SRM-2 provides some better firepower. Of course you can't jump with the SRMs... but that's a minor issue if you have Omnis to carry the Longinus (even though it may still mean you need to spend the SRMs earlier rather than later).
The Longinus can be regarded as the gold-standard really, as the IS Standard, Cavalier and Raiden all have lesser firepower and equal durability (the Raiden does have MRM config if you need more range but since people don't like the +1 to hit penalty, it probably isn't enough to elevate the Raiden over the other standard-likes).
If transportation isn't an issue, the Grenadier has some merit. It has limited mobility but with equal durability (and harder to hit thanks to Stealth Armor) to Inner Sphere standard-type suits, and much more firepower.
Oft seemingly forgotten, there's also the Trinity suits. They date from '66, so they are part of the FedCom Civil War era. At 8 points their armor is pretty good, and they have okay-to-good firepower (especially if TacOps rules for Man-Portable Plasma Rifles are in), but they're limited to ground movement only, which limits their use as genera purpose suits. Good in cities though.
The Gray Death Standard is similar, trading jump jets for increased ground mobility, AP mount and Improved Sensors, making it a good city fighter but not quite a GP suit, perhaps.
Finally, the Fa Shih and Purifier offer neat tricks with their mines and stealth, respectively, but they lack durability a bit, especially the Purifier though its stealth does off-set this to a degree. Good suits, but ultimately they are a bit more specialized. Also in this category is the Infiltrator, which offers good firepower and range with its Magshot but it isn't quite tough enough for general purpose combat suit. The Fenrir gets a mention here too, for its mobility and firepower, though its pitiful armor means it really isn't a general purpose unit.
All in all, i vote for the Longinus.
Longinus BA (King David) w/Tac Ops rule in effect
Do note the suit has a David Gauss Rifle, not King David.