Hmm, out of all those battles I think there was always a non-top tier character in the battle.
When I'm talking TOP TIER, I'm talking people that are almost living legends due to their power. Whitebeard, Mihawk, Sengoku, Garp, 3 Admirals, Shanks, etc. These are people I class as "top tier".
BB Pirates (pre-Shiryuu + 4 ID + gura gura) = "high tier", but not top
Jozu = high, not top
John Giant = ??? (honestly didn't see enough of him, I'd expect high given he's a VA with some renown)
Ace = high, not top
Jimbei = high, not top
Now I'm not saying that any top tier will easily beat a high tier contender. Again I'm going to go with a pokemon analogy (liking them recently), and under the right conditions, an infernape (OU tier) can beat a Ho-Oh (Uber tier). However higher tiers usually have an advantage over the tiers below them, and the potential to OHKO, or at least be unfazed, by those in lower tiers.
Aokoji can OHKO people in a lower tier (Jozu), but still receive noticable damage and not have a relatively "easy" time with him. Similar thing applies across the board a bit.
What I'm saying is that "easy" victory rarely exists within the same tier, but may exist across tiers. No-one should claim that WB would have easily beaten Akainu if he didn't have a stab wound. Heck, I saw no-one else in this war get up after 1, let alone two quakes. John Giant was unable to return after 1, he cannot possibly be classed as top tier. Aokoji, Kizaru, Sengoku and Garp all would have been able to get up after one quake at least.
Whitebeard is like my new measuring stick...