Welcome back to another Foolish review of the hottest stocks as ranked by Motley Fool CAPS. We're looking at the three best-performing industries over the past 30 days and your favorite long and short candidates in each.

How much difference can a week make? Not much, apparently. Travel services firms such as priceline.com (Nasdaq: PCLN) are still gaining altitude, up 56.8% as a group over the past 30 days. But even they fell short of the heights achieved by diagnostic substances manufacturers, whose 64.4% average return scorched everyone to retain this week's top spot.

This week's third-place finisher is an old favorite. Solar power providers, such as SunPower (Nasdaq: SPWR) and budding tweener Yingli Green Energy (NYSE: YGE), are up 33.7% as a group since late March.

According to you, our Foolish readers, the best stocks in these industries to own now -- i.e., those with four or five of the maximum five stars in CAPS -- are:

Company

CAPS Rating

No. of CAPS Ratings

Percent Bulls

30-Day Price Change

Tag

Amtech Systems (Nasdaq: ASYS)

*****

193

99.0%

13.1%

Solar power

Neogen

*****

209

98.1%

(0.5%)

Diagnostic substances

IDEXX Laboratories

*****

154

98.0%

(0.1%)

Diagnostic substances

Meridian Bioscience

*****

413

97.8%

(10.6%)

Diagnostic substances

FPL Group (NYSE: FPL)

*****

576

97.7%

9.3%

Solar power

Sources: Motley Fool CAPS, Yahoo! Finance; current as of 4/20/08.

And your favorite short candidates -- i.e., those rated with one or two stars in CAPS -- are:

Company

CAPS Stars

No. of CAPS Ratings

Percent Bears

30-Day Price Change

Tag

Nymox Pharmaceutical

*

59

49.2%

(7.8%)

Diagnostic substances

QLT (Nasdaq: QLTI)

*

71

47.9%

15.3%

Diagnostic substances

Hoku Scientific (Nasdaq: HOKU)

*

571

46.6%

28.2%

Solar power

Sources: Motley Fool CAPS, Yahoo! Finance; current as of 4/20.

My favorite stock among today's contenders is Amtech Systems, a supplier of equipment for manufacturing solar cells and related devices. CAPS investor pre176 best explained why in December, I think:

If everyone is at war, what better business is there than being an arms dealer? That's why Amtech is in such a good spot right now, all the solar companies are fighting tooth and nail to increase production dramatically. The proof is in the pudding for Amtech, with solar sales up 150% [year over year], and that doesn't include the $17.4 million that they just booked in one order ... nearly matching the previous year's solar income!

Nor does it include the $5 million in follow-on orders that Amtech announced on March 31. But is this really a cheap stock? A 0.59 PEG ratio based on calendar 2008 earnings estimates screams "yes!"

So does Fool EPS100Momentum, who suggested in a response to pre176's pitch that Amtech's intrinsic value is north of $69 per share. I'm not at all sure that's reasonable. Still, even if he were off by 75%, investors who buy now would realize a gain of at least 40%.

But that's my take. What would you do? Would you buy Amtech at today's prices? Let us know what you think by signing up for CAPS today. It's 100% free to participate.

Cap off your day with related CAPS Foolishness:

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Fool contributor Tim Beyers, ranked 15,491 out of more than 97,000 participants in CAPS, didn't own shares in any of the companies mentioned in this article at the time of publication. Find Tim's portfolio here and his latest blog commentary here. The Motley Fool's disclosure policy is hotter than city asphalt in the summer heat.