How to choose one among different types of LPF?
There are 4 main types of LP filters, Butterworth, Chebyshev1, Chebyshev2, and Elliptic filter. Each differs from the other in the oscillations it has, and in which part of the spectre it is.


Butter has the worst amplitude cutoff, eg the widest transition region, but no osclillations;
Cheb1 has better transition range, but has oscillations in the pass region;
Cheb2 is the opposite of Cheb1, has oscillations in the reject region, but not in the pass region;
Elliptic has the narrowest transition, so therefore the best, but has oscillations in both the pass and no-pass zone. 


Heres a picture to help you visualize:

http://i.stack.imgur.com/rJMQw.png

Now, seeing as you need a peek detector, you should use a flat filter for the pass zone, so Butter or Cheb2. I advise Cheb2, because you don't really care about the higher frequencies.

In matlab, the function is cheb2ap, and is used like this:

[z, p, k] = cheb2ap(N, La) 
% where La is the allowed ripple in the stopband (usually around 0-1.5 dB).

% And now the filter transfer function is acquired with:
b = k*poly(z);
a = poly(p); % b is the polynomial for zeroes, a is polynomial for poles
[H w] = freqz(b,a, N)  % N is the number of points for calculation


Now you have frequency response of your filter, H, and vector for its frequency, w, and you can plot the filter with plot(w, abs(H)).

Hope this helps :)