And, all are not equal. The Flak 18 and 36 are the main mulitpurpose variants. The Flak 37 was solely an AA gun and incapable of being easily used in the ground fire role, having revised data feeds and sights. The Flak 41 (the L 71 model) was largely a failure with only a few hundred being built. It had numerous issues with the carriage and barrel before these were worked out. In the end the Flak 41 saw virtually no field service with all but a few being staticly emplaced in defense of the Reich. By mid 1944 just 279 were in service. The reason the 88 saw so much service in the field had to do more with Nazi politics and improvisation thorugh necessity than by design. Almost all of these guns belonged orgainzationally to the Luftwaffe not the Heer. As such, they came under Göring's pervue. The result was that flak units got a higher percentage of motorization than many Heer units did. Göring also saw to it that flak got priority over army artillery and other users of explosives in shell production. The result of this was that more and more frequently as the war progressed it was Luftwaffe flak units that had mobility and ammunition. This meant that they became a substitute for the lack of artillery and shells many Heer units suffered from. So, the 88 became the weapon of choice through a combination of factors none of which had anything to do with the utility of the gun itself.
Perhaps you are right TA, but you can't deny the 88 was particularly effective in its AT role all throughout the war, both as an official AA artillery piece and later as an AT one, and even on the later tanks. So it did is job well and gain a great reputation, overrated? no, it after all is an AA gun that did the job better then its AT counterparts at the time, at least until the better all round 75mm came in as an AT gun.
From what I have read about using the 88 as an artillery piece is that it wasn't the first choice for anyone. It really wasn't heavy enough for truly effective artillery shoots. The 25 pounder, and 105 mm and 155 mm guns were far superior to the 88 in the artillery roll. It's what they were designed to do. That doesn't mean they weren't used effectively, just not as effective as some others. My favorite would be the 16" guns of the Missouri class BB's. They wacked the stew out of the South Pacific Islands.
For best artillery piece, I'd go with the American 155 mm M1 "Long Tom". But me personally I like the German SPA Hummel.
for the sake of clarity but maybe not many Allied servicemen thought they were being hit by the notorious/famous 8.8cm but in reality the rounds were from 10.5cm leichte FH 18/40 with muzzle brake, an excellent and mobile weapon
The 155mm M1 is really just an 155mm M1917 or M1918 on a new carriage and, the M1917 / M1918 is really just the French Canon de 155 Grand Puissance Filloux (GPF) a fine and useful design that started seeing service in 1917.
Just a small comment on "german 88". It gained it's reputation in the beginning of war, destroying enemy tanks. Medium AA gun is not supposed to be near frontline, so it is a bit strange that it happened. Good explanation is in events of beginning of war, when there were situations when german 37mm AT guns, field artillery and even heavy artillery were unable to stop KV or Matilda tanks, and were finally stopped by 88mm Flak. If germans had more capable AT gun or ammo, there would be no need for Flak 88 to engage tanks, and attacks would be stopped sooner. Germans quickly made 7,5cm Pak 40, which served entire war, and it proved capable tank killer.
Why pick it over the US or Italian 90mm guns or the British equivalant? Then there's the question pointed out earlier in this thread. Which 88?
as for Arty piece, i would say that self propelled Artillery Hummel with 150mm was a terror of fortifications. Mobility and a punch... Good combo.