Personally, I say first pick a class you like to play. That alone will make a big difference on how well you can box it. Reverent made some good points. Boxing a tank and Rog seems easier to me than a cleric and using a merc tank, as you can have the Rog autofollow and adjust the tanks position to keep the rogue behind. I use this technique with a War + Rng box also.
Both Rog and Rng can be macroed as well, but the Rog does require more attention to maximize dps, with backstabs having to be from behind for maximum damage (AA for frontal backstabs help, but not as great as from behind), the use of discs, and having to sneak + hide to use some of them. Still alot of it can be macroed, it just requires alot more attention. On my Rng, I usually have two macros. The first is the initial attack macro, that assists, attacks, casts snare and long dots, the second is the repeatable nukes and dots. Well I guess 3, as I have an attack one without the snares or dots for switching targets on multiple pulls.
I generally don't use kick on macros, as I found the low amount of dps it adds was not worth the macro space and increased aggro (rather push the aggro limit with nukes and dots), but I have added it to my macros, and then I usually have another one just to repeat a few kicks between nukes and dots. This requires more attention too, which is another reason I dropped it from a macro, and just do it manually between macros when there is time and no worry.
Both Rog poisons and the Ranger spells and epic slow offer some utility. I always liked the old world Rog slow+snare poisons, but they become ineffective 50+ level, and have the re-apply time which doesn't work well for multiple pulls. The newer procs work ok, but the duration is very short and makes them less useful for multiple pulls, as you'll have to keep switching targets.
Wiz and Mag are both easy to box and both offer some utility. The Wiz can transport and snare, the Mag pets can root or stun. And they can be easily macroed and require little attention, generally. The Mag earth pet with AAs can be a very effective tank without the need to use a merc.
Enc to me seem to take alot of attention, at least to use them effectively. However, if the Enc is the puller as well, along with a cleric box, it is probably very effective. I've just never been a good Enc, although it is one of my favorite classes, I just never have put in the time to be good at it. (I never needed to be good at CC, as the tanks I was with or boxed, could handle multiples easily, and never learned how to use charm effectively). However, from playing with may Enc, boxed and not, it is clear an effective one, can offer alot, and makes pulling much easier (and steady).
Again, it depends on what you like to play. If you like playing a class, you'll generally get good at it, and can use all of their abilities to your benefit. Some classes, of course, have more versatility and/or abilities.
Hope this helps some.
Yther Ore.