Slicehost is ok, their pricing structure ramps up very fast though. MediaTemple is the one I would recommend to someone for a single (or just a few) site instance. If they needed multiple servers with complex config (or just want to lay them out in your own way) either EC2 (multiple clients model: we have 150+ instances working as clients) or MediaTemple CX (complex hosting - we are in the process of purchasing a 200 node cluster with them) would be my recommendations.