There are several different things to consider when picking a linux distribution to use as your primary operating system and the package manager is an important one.

Dpkg is a robust solution to package management.  Dpkg handles dependency issues gracefully and has fewer problems than rpm in general. Dpkg package manager provides tools for dependency conflict resolution, as opposed to being handled by the frontend. 

Dpkg is a great choice for a package manager.

While rpm is the most popular choice in package management it has several problems that hold it back from being the optimal choice.  Rpm has had a rough history of file format conflicts.  Rpm also doesn't necessarily handle dependencies well which could possibly lead to flaky system upgrades.  Rpm is very widely used so software repositories are very easy to find for your distribution.

Rpm provides a useful package management system, however it has had some issues in the past.

Portage provides a powerful package management system that allows cross-platform support through source-code based package management.  The use of source is good in that it allows for code built for the optimizations available on your particular hardware.  The use of source is bad in the sense that install times will be much larger, as compilation can be a daunting task with large packages.  If the user does not mind large install times Portage is an excellent choice for both application execution speed and compatibility. 

Portage is a great choice for users who are somewhat familiar with linux but the compile times could become an annoyance.

Pacman promises to be a comparable package manager, however it is still relatively young in its developmental cycle.  The systems using pacman tend to be relatively new as well (ArchLinux, Frugalware). 

The Pacman package manager would be more suited towards intermediate to advanced linux users wanting bleeding edge software.