American for a large splash and a decent hit. Not too fond of the T92HMC when compared to the M53/M55, though. Slow driving speed combined with the long reload make the tier X (in my opinion) less able to help turn a battle than the tier IX version. By that I mean it's slower to react and change flanks, and makes shooting & scooting a more time consuming process. But the tier IX hits hard, with better mobility and a partial turret. I'd say that puts it on a par with the tier X. The tier VIII arty, though. The M40/43. That's my personal favorite, hands down.
Russian for accuracy and speed. Shoot & scoot with a fairly fast reload, but the damage is "meh" compared to some of the others. You're clicking a lot more to get the same damage, and you'll actually run out of ammo in a match, if you've got enough targets. But that reload ... I think that's a huge plus.
French for mobility and that sweet turret. Hits are most often anemic. Sure, you've got a clip, but there's a decent gap between shells, and anyone who isn't absolute garbage is going to have a crew that can repair the tracks in the time it takes you to get the next shell out. And I'm talking about just the normal repair skill. Also, fairly useless against heavies, completely useless against superheavies. As in "Plink! That one didn't go through!" and a zero damage hit. I prefer the tier IX over the tier X for the French as well, but that's more for the fun factor. You're fast enough to avoid incoming tanks, assuming that you see them coming, and the mobility has made for some interesting matches.
British for a huge splash and potentially some of the biggest hits (on multiple tanks all humping the same rock) due to that splash and the higher trajectory. Shorter range, slow-as-hell shell speed, but the mortar fire makes this the most useful line for a city map. Less so for an open map, but if there's a scout keeping your scout pinned down and theirs is hiding behind some rocks ... well, if they're worth their salt and looked at the team makeup, and noticed that the scout is platooned with a GC, they'd best not put too much stock in that rock they're hiding behind unless it's really tall.
Only have German to tier VII, so I don't comment on that line, other than to say that it's the second least threatening to me when I'm in a heavy or a non-squishy TD, behind the French which have literally hit me for nothing more than stun damage with a direct impact on more than one occasion.
If you're platooned more often than not, I'd say the Russian line, as the rapid rate of fire and fairly fast movement speed can work wonders for getting your platoon mates spotting damage on the way in, and cover fire on the way out. The smaller splash radius and accurate gun also make it more viable for close arty support, but I say this as a very strong warning against doing so for anyone other than someone you're platooned with. Nobody likes getting a "friendly" derp from above.
On my personal opinion, I'd say the US and/or Russian, then Brit.