Honestly, if you only want to use past results to decide who the better team is then that's what the ICC rankings are for. Maybe I'm being a bit (or a lot, even) contradictory in saying this given my firm stance on how a player's rating should be determined purely on what he's actually done at all levels and that potential should be left out of it, but I tend to look at teams a lot differently. As has been pointed out ad nauseum, it's all just opinion if you look at it any other way, but I've never been a great fan of comparing teams based on what they've done over 18 months or whatever for the purposes of anything other than official rankings.
I think the fact everyone is stumbling and bumbling around with Pakistan is that they have a lot more potential to upset really good teams but that their mean performance is still only around the same level as NZ/WI. Sure they may draw a series against Australia but the nature of the team makes it IMO by the far the most likely to go down to Bangladesh (mass player strikes excepted) or get absolutely pumped by one of the the other two. In fact, if all three sides played ten Tests home and away against each other right now (in a hypothetical universe where you could play sixty Tests in the blink of an eye
) I think Pakistan may run last on the league table because they'd win some games by massive amounts but also put in some shockers and crumble under pressure in close ones. They have the ability to knock off the big guns but they have a lot of inexperienced players which always breeds inconsistency in a side, not to mention intrinsically inconsistent players, no real captain to speak of and a much stronger bowling attack than its batting lineup .. all factors which contribute to what is basically Test cricket's bipolar entity. I've voted for them anyway though because I think that overall they'll do better than the others when you factor in their giant-killing potential - there are certain series you can just rule out WI/NZ winning that I think Pakistan would still be a chance in.
The West Indies lineup at full strength would IMO be the best of the lot if they played every game - not by a lot - but by enough. However they have very little depth in their fast bowling ranks and, crucially, have a ridiculously injury-prone attack. It's extremely rare for Edwards to be fit while Taylor isn't much better and Roach too has shown signs of lacking the requisite fitness for the daily grind of Test cricket. As such, their backup bowlers become a lot more relevant than those of Pakistan and New Zealand and unfortunately they immediately start turning to Rampaul, Pascal, Sammy, Bess etc who are so far from Test standard opening bowlers that it isn't funny. West Indies at full strength is a purely hypothetical entity and the true crux of the team is one that contains Ravi Rampaul for most of his appearances and sometimes bowlers even worse than he. West Indies have a decent team in theory but it never eventuates and the depth in the areas in need is very poor so they'll struggle a bit unless they can improve the fitness of their bowlers or Russell can replicate what he's been doing for the A team of late for the senior side.
New Zealand don't have don't have the unusual problems the other two teams have but they do have the one thing that has held cricket teams back since the birth of the game - substandard players. In the past I haven't subscribed to the whole "New Zealanders are untalented and get by on team morale, good coaching etc etc" line of thinking but in comparing them to West Indies and Pakistan at the current time I actually do think there's some truth in that stereotype for once. The top order is frankly terrible at the moment and I don't think you could conclusively say they had a better batting lineup than Pakistan now that Hopkins has a play as well, and their bowling lineup certainly isn't as good. However, they are a lot more professional and consistent in their cricket and they have more bottle for the close ones than Pakistan (not saying they aren't prone to the odd second innings collapse themselves, mind..), and with Bond and Oram now retired they don't have any persistent injury problems they need ready backups for at all times (other than perhaps Ryder's groin which is becoming a problem).
With the West Indies at full strength I'd rate New Zealand a bit behind but considering the likelihood of someone like Rampaul, Pascal or Dowlin making an appearance to cover an injured player I'd rate them about on par, fractionally below Pakistan.