I voted for stock.
I think swelt maps are ok. They are not absolutely horrible as some of the ones I have seen out there, but I think Phrantic in particular is too limited a map. It is very small and it is not very balanced. It seems like it has too much stuff going on for such a small map - it got 2 rls and 2 armors (compared to just one rl and one armor for much bigger maps like Frag chamber or Delta) and a mega every minute. Also, it seems like Phrantic is too easy to control. And the map is so small that pretty much almost every spawn can turn into a spawn kill very quickly. It takes very little time to get from one point on the map to even the most remote point on the map, so that is bad news for the guy who just spawned. Hence Phrantic has this large luck factor, where one player can beat the other one 10-0 and then in the next game the other guy does the same to the first guy. Happens all the time on Phrantic - sometimes something like that happens in one game - one guy gets 9 kills, then he gets killed and the other guy puts together a huge comeback. So luck is clearly a bigger factor in phrantic than it is in many of the other maps, and that is why many players don't like it.
Never outgunned is a bit more balanced and is a bit harder to control. Also, unlike Phrantic, which is made for rl, Never outgunned allows for some chaingun -friendly areas, and that makes it a slightly better map than phrantic.
But still, the default maps rock.
Edge is a classic map. You may dislike other maps and avoid them, but to be considered any good at D3, being decent on the Edge is a must. Edge is just about the only map in D3 which is impossible to control (at least in the sense of running the items), so things like strategy take on a greater importance. Edge is also versatile, as it can be used for TDM.
Then comes Delta Labs. I believe this is the single most underrated map in the game. Very few people play it these days and this map should definitely be played more. But I have seen the QCON demos of pros playing this map and I have to say, it was a thing of beauty, compared to say, rocketboy camping armor in Frag Chamber during better part of the Final match

I think ability to play this map well is also a sign of a good player.
Then there is Frag chamber. I am not particularly good at it, but it is still a pretty good map, especially compared to Swelt. Yes, on Frag chamber, like on other default maps you can occasionally get spawn killed, but it is never on the scale that it seems to happen on Swelt maps. So on Frag chamber, as on the two maps above, luck is not nearly as big a factor, so the better player usually ends up winning.
Lights out is also pretty good. It is the most Swelt-like of the default maps - given its small size, more chances for spawn raping. The fact that it has narrow rooms makes rl very effective, which also makes it more like Swelt. So on Lights out, the luck is much more of a factor than on other default maps, but still, I think this map has more balance and less randomness in the results than Phrantic.
I think some people almost consider Aerowalk a Swelt map - because it is so small, but its not

I think the jury is still out on Aerowalk - I think too few people can play it well, but it sure doesn't look like a map that can be as good as default, but it is somewhat strange it is not played more, compared to how much people play phrantic.
Actually, I remember me and keycode testing Rustbucket, which is the third Swelt map from the same set as Phrantic and Never outgunned. From what I have seen, it may have been the best Swelt map of the three. I may be wrong, as I only played like a couple of games on it, but still it is ironic that this map is never played.
Also, some other maps line Mancer deserve at least a try. It beats playing Phrantic 24/7. Also, I look forward to The Lost Fleet for D3 - it was like the best map out of default maps for Quake 4. Though I am a bit worried that with D3 rl being as dominant as it is, and with The Lost Fleet being a pretty small map, it may turn into another Swelt clone. That would be the case especially if the rl was at the same place as in q4 - then it would be such a spawn kill fest that would probably surpass Phrantic. I was thinking maybe rl should not be in The Lost Fleet for D3 at all - that would give the map more balance and some interesting dynamics.