The first critical thing is getting one the proper size. The tendency for ages has been to do a rough calculation (or just look at what was there) and replace it with one the same or larger - this is rarely the proper solution. Any boiler will only reach its efficiency specs steady state...each time it starts then shuts off, it wastes some energy and it takes awhile to get up to max efficiency, if it ever does. Short cycles are death to a combustion device and a hit to your wallet as well.
There are a bunch of decent boilers out there, but one critical thing is to find one that is well supported by parts and technical expertise. Any boiler needs to be installed and set up properly, or it will not meet your expectations. All that being said, when I looked around to replace mine a few years ago, I called a bunch of contractors, got a bunch of opinions, then decided on a Buderus unit. The US factory/distributor is about 15-miles from my house, which didn't hurt, either (in Londonderry, NH).