My personal choice was the E420
I've enjoyed mine for over four years now. It's my primary commute car (70 miles round trip), so it racks up the miles quickly. I have over 110,000 on her now and she's still running perfectly. When I was considering a W124, I test drove both E320 and E420, and to me, the choice was clear. The added power of the E420 was the deciding factor for me. The E420 in stock form is just about perfect except for the consensus that the stock tires are too small. I did a subtle +1 upgrade to 16" rims and wider tires. It did wonders for the car's handling.
The slightly nicer interior of the E420 was a bonus.
1995 Mercedes E420
1990 Mercedes 500SL