Allergan (NYSE:AGN) is one of the most underappreciated big pharma stocks on the market right now. That's the thought that occurred to me as I listened to Brent Saunders, Allergan's CEO, speak at the Wells Fargo Healthcare Conference on Thursday.

One thing's for sure, though: Underappreciated stocks don't stay that way for too long. Investors eventually begin to see the value in the stock, interest increases, and the stock moves higher. I think that's going to happen with Allergan. Here are three top reasons investors should consider buying this big pharma stock right now.

Green "buy stock" button on computer keyboard

Image source: Getty Images.

1. It's a bargain

Allergan stock trades at less than 13 times expected earnings. That alone gives the stock a more attractive valuation than many big pharma stocks. But there's an even better reason Allergan is a bargain: its growth potential.

The consensus among Wall Street analysts is that Allergan will grow earnings by more than 13% annually over the next five years. Is that doable? I think so -- and I'll point out why shortly. Factoring in that rate of growth, Allergan's PEG ratio is only 1.06. There are only three large-cap biopharma stocks on the market that have lower PEG ratios -- AbbVie, Celgene, and Shire.

2. Positioned well for the long run

Why is Allergan positioned well for growth over the long run? Let's start with its medical-aesthetics business. Botox continues to be a rock-solid product for Allergan, with sales increasing 13.5% year over year in the second quarter. Juvederm sales jumped more than 17% higher than the prior-year period. The company also acquired Zeltiq Aesthetics earlier this year, a move that bolsters its lineup in medical aesthetics.

Consumer penetration for Allergan's medical-aesthetics products is really low -- between 5% and 10%. The company sees the market for men and millennials as great growth opportunities. 

There is also huge potential for what Allergan calls its "six stars." These pipeline candidates include migraine drugs ubrogepant and atogepant, and uterine-fibroids drug Esmya. Two late-stage programs that intrigue me even more, though, are abicipar and cenicriviroc (CVC). Abicipar could be one of the most powerful treatments for age-related macular degeneration ever, while CVC holds promise in treating what some have dubbed the "next hepatitis C" -- non-alcoholic steatohepatitis. 

3. Bet on the jockey

You've no doubt heard the old saying "Bet on the jockey, not on the horse." My view is that both jockey and horse are important. Allergan is the horse, while CEO Brent Saunders is the jockey. I think both are good bets.

I respect Saunders for his leadership on drug pricing with Allergan's social contract announced last year. His is a voice of sanity that balances responsibilities to shareholders with acknowledging the important role drugmakers have in society. But I also think Saunders has made some very smart business moves.

Selling off the Actavis generics business to Teva (NYSE:TEVA) was one of them. While it was a good deal for Teva, it was even better for Allergan. The company should be able to achieve more success focusing on specialty drugs.

Another smart move, in my view, was Saunders' decision to partner with Editas Medicine (NASDAQ:EDIT) to develop CRISPR gene-editing drugs for eye diseases. Allergan became the first big pharma to collaborate with the small gene-editing pioneer earlier this year. In January, Saunders said he thought the biopharmaceutical industry was "entering what could be a golden age" with advances including CRISPR genomic editing. I think he's right -- and that the alliance with Editas could pay off tremendously over the next few years.

The wild card

The wild card that could really change the dynamics for Allergan is the prospects for acquisitions. Allergan's previous growth stemmed largely from making deals. Saunders hasn't been shy about scooping up smaller companies, including Zeltiq earlier this year. He mentioned at the Wells Fargo conference that Allergan is likely to make more bolt-on acquisitions.

However, Saunders also said he thinks "there will be another wave of consolidations" in the industry. He added that he'd like for Allergan to "be on the offense" with respect to this wave. That could mean bigger deals lie ahead.