I think IBM is the best among all these. It comes with light path technology so there are no problems at the time of minor failures.
i work at reynolds and reynolds and all of our servers in the field are ibm's now, and i have to say the light path is pretty sweet. The only thing i would avoid are the ibm x305 servers. their 4th system fans SUCK. but luckily they are useless even when they work. big thumbs up for the 306m 245 and 246 and the new 3250 servers are pretty kick a$$ as well... personally i build my servers from the ground up, except for one which is a dell 2650 and i have no complaints about it
HP- Ok for most midrange apps
IBM- mid to upper range, DB intensive apps
Dell- naeh...Seen too many die...
Sun- Very good in special apps, their new X64 Servers are amazing, great for hosting
Appro- High Performance Cluster? Cloud Computing? You got it!