I rank Larry Allen with John Hannah, Anthony Munoz, and Jim Parker as the best OLs to ever play the game. Center would be a toss up between Mike Webster, Jeff Van Note, and Jim Otto. Pick one and I would be happy with him.
How's that for an all-time O-Line?
LT-Munoz, LG-Hannah, C-Webster, RG-Allen, RT-Parker
As someone else said, if Erik Williams wouldn't have gotten hurt in that car crash he could have been the greatest RT of all-time and replaced Parker on my line.
As for Hannah vs Allen, Hannah could do some things that Allen couldn't and vice versa. Hannah was very strong and could dominate his opponent but he was also an excellent technician and would routinely block 2-3 guys on a play as he would move from one level to another. He was very quick and excelled at pulling and blocking in space throughout his career. His vision and ability to read made him the best in his time. A pulling Guard is like a RB when it comes to needing good vision and balance but instead of avoiding contact, he uses it to take guys out. Hannah was the best I ever saw at this.
Allen lost a lot of his mobility later in his career but early on he amazed the world running down a defender who had picked off a pass. No one in NFL history was a strong as Allen IMO. His strength is amazing and the power he displays is without equal. Watching DTs try to avoid him was funny at times, especially later in games when they just didn't want to take that shot to the chest anymore. You would see them stunting a lot, trying not to face him. Allen could get sloppy at times though and tended to rely too much on his strength and not enough on proper footwork and technique. But in his prime he was the best ever.
Munoz is simply the best OT to ever play the game and is without equal IMO. Even Big-E in his prime wasn't as good as Munoz was, he was stronger but nowhere near as good technically.