I've been to Vegas several times and for different types of trips (business, stags, fun with buddies, fun with wife, paintball tournaments, etc) so with that in mind, here's my general opinions on the places I've stayed:
NYNY
This used to be my number one choice. The Casino is smaller then most, but I love the shops and restaurants. The Mexican and steak restaurants are top notch. Drinks used to flow very freely when playing the slots and they use real orange juice for screwdrivers. However, drink service slowed down the last time I was there. Also, the last time (and I mean LAST time) we stayed there, we had dirty sheets on one of the beds and 2 days into our stay, we got dirty towels. I was not impressed and have recommended to everyone to steer clear.
MGM
The last time my wife and I stayed there we swore would be our last. I used to like it, but the renovations moved the sports betting area from a nice quiet spot, to sandwiched between the poker room and one of the bars. When we were there last (2 years ago), the whole place looked like it needed renovating again. It seemed like 1/3 of the slots had lights that were out, our room had a radio clock that was possessed, and the phone didn't work (visits to the Front Desk had no effect, except on the last night they removed the broken phone ... didn't replace it). Walking through there a few months ago, they have done work on the slots to improve things. I'm just not sure if I'd risk staying there again.
Excalibur
Cheap. It's got a fun theme and it's cheap. Not great food and drinks are average, but it's cheap. Did I mention it was cheap?
Bellagio
Beautiful resort! I love the theme and the garden area. The cafe is someplace I always eat at when in Vegas. It's away from the sound of the Casinos, foods great, and reasonably priced ... it's a nice oasis of quietness after a couple days of sensory overload. However, drink service is slow and they use Sunny-D for screwdrivers (seriously, a 5* resort that uses Sunny-D!!). Also, there's been a significant increase in the number of working girls ... a sign of the economic times.
Treasure Island
It's okay, the new owners are trying to make it better, but pricing doesn't match what you get. Expect breakfast here to run you $20/person which is ludicrously expensive. Drink service is average. The new outside show, Sirens, is a waste of time. The original show was much better. The nice thing about TI is it's location to the Fashion Mall.
Venetian
I used to hate the Venetian. I tried very much to love it (it's decor is second only to the Bellagio and then arguably so) but their service used to be horrid but it has improved so well that I've stayed there the last two times I was in Vegas. Drink service is above average, and excellent if you're playing video poker at the bars. Rooms are much larger then any of the others I've stayed at (I've been told their comparable to Caesars). Restaurants are excellent. The only bad thing about it is it's location so far back from the Strip and lack of monorail service.
Places I haven't stayed but know a bit about:
City Centre (Aria, etc)
I haven't stayed here but love walking through the Casino. It's got a coffee/chocolate theme to it and is very different from other Casinos. It's well worth checking out. You can find some deals there but it's usually around $120+/night and goes up significantly.
Cosmopolitan
It's right beside the new City Centre and is often thought to be part of it, but it isn't. You HAVE to do a walk around all levels of the Cosmopolitan! After so many trips to Vegas, walking in through here is the one place that still wows me because it's so over-the-top (3 story chandelliers and 6-foot high stilletos). Also, they have a little burger bar right on the strip that is cheap and great. I'd love to take the wife here to stay, but we're more a jeans and t-shirt couple then $3k suit and evening gown couple. It's still fun to check out though!
Mirage
I have always like the garden entry to the Mirage but I'm not a fan of the renovations to the Casino area. The burger bar is a must (alcoholic shakes and great burgers), as is the Rumbar (slushy alcoholic drinks). The Brazilian restaurant is also really really good.
Hope some of this helps! You'll have a fun time anywhere you stay, just avoid the deep-fried twinkies in Old Town!